He won the NCAA title while at Ohio State in the 800 m in 1948 and 880 yd (800 m) in 1949. After leaving the university he won the AAU title from 1949 to 1951 at 800 m, in 1953 and 1954 at 880 yd (800 m) and in 1952 at 400 m. He also won the 800 m at the 1951 Pan American Games in Buenos Aires, Argentina.
Whitfield's most notable achievements, however, may have been as an Olympic athlete. At the 1948 Olympics in London, Whitfield won the 800 m and was a member of the winning 4 × 400 m relay team. He also earned a bronze medal in the 400 m. At the 1952 Olympics in Helsinki, Finland, he repeated his 800 m victory. He also earned a silver medal as a member of U.S 4 × 400 m relay team. He set a world record in 880 yd (800 m) of 1:49.2 in 1950 and dropped it to 1:48.6 in 1952.
In his 47 years in Africa, Whitfield trained and gave consultation to dozens of athletes who represented their countries as Olympians and All-Africa Games champions. Whitfield also arranged sports scholarships for over 5,000 African athletes to study in the United States.
During his career as a diplomat extraordinaire, he traveled to over 132 countries and played a key role in training and developing African athletes. The late U.S. President Ronald Reagan wrote of him: "Whether flying combat missions over Korea, or winning gold medal after gold medal at the Olympics, or serving as an ambassador of goodwill among the young athletes of Africa, you have given your all. This country is proud of you, and grateful to you." Shortly after his retirement from government service in 1989, "Marvelous Mal" was invited to the Oval Office, where President George H.W. Bush recognized his exemplary service to the nation and the world.