Form 497K NORTHERN FUNDS

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NORTHERN FUNDS Mid Cap Index Fund

INVESTMENT OBJECTIVE

The Fund seeks to provide investment results approximating the overall performance of the common stocks included in the Standard & Poor’s
MidCap 400^® Composite Stock Price Index (“S&P MidCap 400 Index” or “Index”).

FEES AND EXPENSES OF THE FUND

This table describes the fees and
expenses that you may pay if you buy, hold, and sell shares of the Fund. You may be required to pay commissions and/or other forms of compensation to a financial intermediary for transactions in Shares Class shares, which are not reflected in
the tables or the examples below.

EXAMPLE

The following Example is
intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of
those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same (taking into account the expense reimbursement arrangement for one year). Although your actual
costs may be higher or lower, based on these assumptions your costs would be:

PORTFOLIO TURNOVER. The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction
costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual portfolio operating expenses or in the Example, affect the Fund’s performance. During the most recent fiscal
year, the Fund’s portfolio turnover rate was 15.17% of the average value of its portfolio.

PRINCIPAL INVESTMENT STRATEGIES

Under normal circumstances, the Fund will invest substantially all (and at least 80%) of its net assets in equity securities included in the S&P MidCap
400^® Index, in weightings that approximate the relative composition of securities contained in the Index, and in S&P MidCap 400 Index futures approved by the Commodity Futures Trading
Commission.

The Index is a free float-adjusted market capitalization index consisting of 400 mid-capitalization stocks. As of May 31, 2022, the
approximate market capitalization of the companies in the Index was between $1.57 billion and $17.94 billion. It is rebalanced quarterly. The Fund generally rebalances its portfolio in accordance with the Index.

NTI uses a “passive” or indexing approach to try to achieve the Fund’s investment objective. Unlike many investment companies, the Fund does
not try to “beat” the index it tracks and does not seek temporary defensive positions when markets decline or appear overvalued. NTI will buy and sell securities in response to changes in the Index as well as in response to subscriptions
and redemptions.

The Fund generally invests in substantially all of the securities in the Index in approximately the same proportion as the Index (i.e.,
replication). In certain circumstances, however, the Fund may not hold every security in the Index or in the same proportion as the Index, such as to improve tax efficiency or when it may not be practicable to fully implement a replication strategy.
Rather, it will use an optimization strategy to seek to

construct a portfolio that minimizes tracking error versus the Index while managing transaction costs and realized capital gains and losses.

The Fund intends to be diversified in approximately the same proportion as the Index is diversified. The Fund may become “non-diversified,” as
defined in the Investment Company Act of 1940 (the “1940 Act”), solely as a result of a change in relative market capitalization or index weighting of one or more constituents of the Index. A “non-diversified” fund can invest a
greater percentage of its assets in a small group of issuers or in any one issuer than a diversified fund can. Shareholder approval will not be sought if the Fund becomes non-diversified due solely to a change in the relative market capitalization
or index weighting of one or more constituents of the Index.

In seeking to track the performance of the Index, from time to time the Fund may focus its
investments (i.e., invest more than 15% of its total assets) in one or more particular sectors. As of March 31, 2022, the Fund focused its investments in the industrials sector.

NTI expects that, under normal circumstances, the quarterly performance of the Fund, before fees and expenses, will track the performance of the Index within
a 0.95 correlation coefficient.

The Index is created and sponsored by S&P Dow Jones Indices (“S&P”), as the index provider. S&P
determines the composition and relative weightings of the securities in the Index and publishes information regarding the market value of the Index. S&P does not endorse any of the securities in the Index. It is not a sponsor of the Mid Cap
Index Fund and is not affiliated with the Fund in any way.

PRINCIPAL RISKS

As with any investment, you could lose all or part of your investment in the Fund, and the Fund’s performance could trail that of other investments. The
Fund is subject to certain risks, including the principal risks noted below, any of which may adversely affect the Fund’s net asset value (“NAV”), total return and ability to meet its investment objective. Each risk noted below is
considered a principal risk of investing in the Fund, regardless of the order in which it appears. The significance of each risk factor below may change over time and you should review each risk factor carefully.

MARKET RISK is the risk that the
value of the Fund’s investments may increase or decrease in response to expected, real or perceived economic, political or financial events in the U.S. or global markets. The frequency and magnitude of such changes in value cannot be predicted.
Certain securities and other investments held by the Fund may experience increased volatility, illiquidity, or other potentially adverse effects in response to changing market conditions, inflation, changes in interest rates, lack of liquidity in
the bond or equity markets, volatility in the equity markets, market disruptions caused by local or regional events such as war, acts of terrorism, the spread of infectious illness (including epidemics and pandemics) or other public health issues,
recessions or other events or adverse investor sentiment or other political, regulatory, economic and social developments, and developments that impact specific economic sectors, industries or segments of the market. These risks may be magnified if
certain events or developments adversely interrupt the global supply chain; in these and other circumstances, such risks might affect companies worldwide due to increasingly interconnected global economies and financial markets.

MID CAP STOCK RISK is the risk that
stocks of mid-sized companies may be subject to more abrupt or erratic market movements than stocks of larger, more established companies. Mid-sized companies may have limited product lines or financial resources, and may be dependent upon a
particular niche of the market.

TRACKING RISK is the risk that the Fund’s performance may vary from the performance of the index it tracks as a result of share purchases and redemptions, transaction costs, expenses and other factors.

SECTOR RISK is the risk
that companies in similar businesses may be similarly affected by particular economic or market events, which may, in certain circumstances, cause the value of securities of all companies in a particular sector of the market to decrease.

INDEX
RISK is the risk that that the Fund would not necessarily buy or sell a security unless that security is added or removed, respectively, from the Index, even if that security
generally is underperforming, because unlike many investment companies, the Fund does not utilize an investing strategy that seeks returns in excess of the Index. Additionally, the Fund rebalances its portfolio in accordance with the Index, and,
therefore, any changes to the Index’s rebalance schedule will result in corresponding changes to the Fund’s rebalance schedule.

DERIVATIVES RISK is the risk that derivatives may pose risks in addition to and greater than those associated with investing directly
in securities, currencies and other instruments, may be illiquid or less liquid, more volatile, more difficult to value and leveraged so that small changes in the value of the underlying instrument may produce disproportionate losses to the Fund.
Derivatives are also subject to counterparty risk, which is the risk that the other party to the transaction will not perform its contractual obligations. The use of derivatives is a highly specialized activity that involves investment techniques
and risks different from those associated with investments in more traditional securities and instruments.

NON-DIVERSIFICATION RISK Under the 1940 Act, a fund designated as “diversified” must limit its holdings such that the securities of issuers which individually represent more than 5% of its total assets must in
the aggregate represent less than 25% of its total assets. The Fund is “diversified” for purposes of the 1940 Act. However, in seeking to track its Index, the Fund may become “non-diversified,” as defined in the 1940 Act, solely
as a result of a change in relative market capitalization or index weighting of one or more constituents of the Index. A non-diversified fund can invest a greater portion of its assets in the obligations or securities of a small portion of its
assets in the obligations or securities of a small number of issuers or any single issuer than a diversified fund can. In such circumstances, a change in the value of one or a few issuers’ securities will therefore affect the value of the Fund
more than if it was a diversified fund.

As with any mutual fund, it is possible to lose money on an investment in the Fund. An investment in
the Fund is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.

FUND PERFORMANCE

The bar chart and table that follow provide an
indication of the risks of investing in the Fund by showing (A) changes in the performance of the Fund from year to year, and (B) how the average annual total returns of the Fund compare to those of a broad-based securities market index.

The Fund’s past performance, before and after taxes, is not necessarily an indication of how the Fund will perform in the future.

Updated performance information for the Fund is available and may be obtained on the Fund’s website at northerntrust.com/funds or by calling
800-595-9111.

CALENDAR YEAR TOTAL RETURN*

* Year to date total return for the six months ended June 30, 2022 was (19.64)%. For the periods shown in the bar chart
above, the highest quarterly return was 24.29% in the fourth quarter of 2020, and the lowest quarterly return was (29.69)% in the first quarter of 2020.

AVERAGE ANNUAL TOTAL RETURN

(For the periods ended December 31,
2021)

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the
impact of state and local taxes. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements,
such as 401(k) plans or individual retirement accounts.

In calculating the federal income taxes due on redemptions, capital gains taxes resulting from
redemptions are subtracted from the redemption proceeds and the tax benefits from capital losses resulting from the redemptions are added to the redemption proceeds.

MANAGEMENT

INVESTMENT ADVISER AND PORTFOLIO MANAGERS. NTI, an indirect subsidiary of Northern Trust Corporation, serves as the investment adviser of the Mid Cap Index Fund. Lucy A. Johnston, Vice President of NTI, and Brent D. Reeder, Senior Vice
President of NTI, have been managers of the Fund since July 2019 and November 2006, respectively. The Northern Trust Company, an affiliate of NTI, serves as transfer agent, custodian and sub-administrator to the Fund.

PURCHASE AND SALE OF FUND SHARES

You may purchase Shares
Class shares of the Fund by opening an account directly with Northern Funds (the “Trust”) with a minimum initial investment of $2,500 in the Fund ($500 for an IRA; $250 under the Automatic Investment Plan; and $500 for employees of
Northern Trust and its affiliates). The minimum subsequent investment is $50 (except for reinvestments of distributions for which there is no minimum). The Fund reserves the right to waive these minimums. You may also purchase Shares
Class shares of the Fund through an account at Northern Trust (or an affiliate) or an authorized intermediary.

If you purchase, sell (redeem) or
exchange Shares Class shares through an authorized intermediary, you may be required to pay a commission and/or other forms of compensation to the intermediary. In addition, an authorized intermediary may impose different investment minimums
than those set forth above. The Fund is not responsible for any investment minimums imposed by authorized intermediaries or for notifying shareholders of any changes to them.

On any business day, you may sell (redeem) or exchange shares through your account by contacting your Northern Trust account representative or authorized
intermediary. If you

purchase shares directly from the Trust, you may sell (redeem) or exchange your shares in one of the following ways:

TAX INFORMATION

The Fund’s distributions are generally
taxable to you as ordinary income, qualified dividend income, capital gains, or a combination of the three, unless you are investing through a tax-exempt or tax-advantaged arrangement, such as a 401(k) plan or an individual retirement account.
Distributions may be taxable upon withdrawal from tax-advantaged accounts.

PAYMENTS TO BROKERS-DEALERS AND OTHER FINANCIAL INTERMEDIARIES

If you purchase the Fund through a broker-dealer or other financial intermediary (such as a bank), the Fund and its related companies may pay the
intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the broker-dealer or other intermediary and your salesperson to recommend the Fund over another investment. Ask your
salesperson or visit your financial intermediary’s website for more information.