A 1950 graduate of McClymonds High School, where he was a star athlete in several sports, Marion Sims became physical director for the Meltzer Unit of the Oakland, California, Boys Clubs in 1956. In 1963, after studying Boys Club administration at New York University, Sims returned to the Oakland Boys Club as unit director, a position he held for two years before becoming executive director. Sims served as executive director until his retirement in 1992.

In his long career with the Boys & Girls Club Movement, Sims distinguished himself through extraordinary leadership. He served as president of the Northern California Boys Club Association, chairman of the Pacific Administrative Conference, chairman of the human resource committee of Boys & Girls Clubs of America and a national board member. In recognition of his contributions, Sims received the Boys Club Distinguished Service Award and was named "Professional of the Year" in 1987 by the Pacific Region.

In addition to his Boys & Girls Club work, Sims has been affiliated with numerous civic organizations, including the Lions Club of Oakland, the Rotary Club and the Mayor's Council on Youth Opportunity.