Forbes is famous for his theme paintings, posters and prints of various PGA and Seniors golf tournaments. He has designed more than 20 commemorative postage stamps for the U.S. Postal Service, including the 1988 Olympic Stamps, Lou Gehrig and Jesse Owens stamps and the "America the Beautiful" series. His work has appeared in countless magazines, including Sports Illustrated and TIME, and corporate clients include Exxon, Pepsi-Cola, ABC-TV, Eastman Kodak, General Electric, American Airlines, and the NFL.
In 1988, Forbes was selected as the Official Artist for the Seoul Olympics. His paintings of the events are housed in the Olympic Museum in Seoul. In addition, he has painted official posters for the Boston Marathon, The Americas Cup, Indianapolis 500, and the 1992 and 1996 Olympic games.