In May, 1964, she made the Fort Lauderdale New's "Girls All-County Team" as a Sophomore swimming for Pine Crest in the 200-yard Individual Medley with a 2:23.7, and the 100-yard butterfly with a 1:00.4.

Future accomplished distance swimmer Diana Nyad was a Pine Crest teammate in her Junior year who made the All County list in the 100-yard backstroke.

Corson's first high level competition was at the 1966 British Empire and Commonwealth Games, where she swam in the 4 x 100-meter medley relay where she won a silver medal.

[2] At the 1968 Olympics in Mexico City, Corson won a bronze medal as a member of Canada's third-place team in the women's 4x100-metre freestyle relay, together with teammates Angela Coughlan, Elaine Tanner and Marion Lay.

[2] Marilyn noted that her Olympic preparation consisted of an average of 22 hours a week of training broken into 11 work outs.

She finished seventh in Munich in the 4x100-meter medley relay swimming with Leslie Cliff, Wendy Cook-Hogg, and Sylvia Dockerill.

[2] She married Mike Whitney, a student from Greenville, Michigan, on September 8, 1972, in Munich after Olympic competition was completed.

[9] She began a Ph.D. program at Virginia Polytechnic Institute and State University in 2003, where she taught as a Graduate Teaching Assistant.