American Century Mutual Funds, Inc.

Prospectus Supplement

Select Fund

Supplement dated April 15, 2009¡ Prospectus dated March 1, 2009

The following replaces The Fund Management Team section on page 11 of the prospectus.

The Fund Management Team

The advisor uses teams of portfolio managers and analysts to manage funds. The teams meet regularly to review portfolio holdings and discuss purchase and sale activity. Team members buy and sell securities for a fund as they see fit, guided by the fund’s investment objective and strategy.

The portfolio managers on the investment team who are jointly and primarily responsible for the day-to-day management of the fund are identified below.

Keith Lee

Mr. Lee, Vice President and Portfolio Manager, has been a member of the team that manages the fund since 2001. He initially joined American Century Investments in 1998 and rejoined in 2001. He became a portfolio manager in 2003. He has a bachelor of science degree in industrial engineering from ColumbiaUniversity. He is a CFA charterholder.

Michael Li

Dr. Li, Vice President and Portfolio Manager, has been a member of the team that manages the fund since 2003. He joined American Century Investments in 2002 as an investment analyst and became a portfolio manager in 2006. Before joining American Century Investments, he attended The Wharton School of Business, University of Pennsylvania, where he obtained his MBA. He also has a bachelor of science degree from the University of Science and Technology of China and a Ph.D. from the University of Michigan.

The statement of additional information provides additional information about the accounts managed by the portfolio managers, the structure of their compensation, and their ownership of fund securities.

American Century Investment Services, Inc., Distributor

©2009 American Century Proprietary Holdings, Inc. All rights reserved.

CL-SPL-65461   0904

American Century Mutual Funds, Inc.

Prospectus Supplement

Ultra® Fund

Supplement dated April 15, 2009¡ Prospectus dated March 1, 2009

The following replaces The Fund Management Team section on page 11 of the prospectus.

The Fund Management Team

The advisor uses teams of portfolio managers and analysts to manage funds. The teams meet regularly to review portfolio holdings and discuss purchase and sale activity. Team members buy and sell securities for a fund as they see fit, guided by the fund’s investment objective and strategy.

The portfolio managers on the investment team who are jointly and primarily responsible for the day-to-day management of the fund are identified below.

Keith Lee

Mr. Lee, Vice President and Portfolio Manager, has been a member of the team that manages the fund since 2008. He initially joined American Century Investments in 1998 and rejoined in 2001. He became a portfolio manager in 2003. He has a bachelor of science degree in industrial engineering from ColumbiaUniversity. He is a CFA charterholder.

Michael Li

Dr. Li, Vice President and Portfolio Manager, has been a member of the team that manages the fund since 2008. He joined American Century Investments in 2002 as an investment analyst and became a portfolio manager in 2006. Before joining American Century Investments, he attended The Wharton School of Business, University of Pennsylvania, where he obtained his MBA. He also has a bachelor of science degree from the University of Science and Technology of China and a Ph.D. from the University of Michigan.

The statement of additional information provides additional information about the accounts managed by the portfolio managers, the structure of their compensation, and their ownership of fund securities.

American Century Investment Services, Inc., Distributor

©2009 American Century Proprietary Holdings, Inc. All rights reserved.

CL-SPL-65460   0904