• President and Executive Director of the Proteus Fund
• Founder of the Peace Development Fund
• Active in the environmental, anti-war, and campaign-finance-reform movements

Margaret Gage is the President and Executive Director of the Proteus Fund, which she established in 1995. She also founded the an arms control group called the Peace Development Fund in 1981 and served as its Executive Director for eleven years. She was the Executive Director of the Ottinger Foundation from 1992 through 1999, and is currently a board member of both the Women's Fund of Western Massachusetts and the Council for a Livable World, the latter being a leftwing advocate of arms reduction and an opponent of the national missile defense system.

In September 2004, Gage was one of more than fifty anti-President Bush signatories to a political letter crafted by a group called United Progressives for Victory (UPV). Addressed to supporters of then-presidential candidate Ralph Nader, the UPV letter urged its recipients not to vote for Nader, who had no realistic chance of winning the election, but to cast their ballots instead for John Kerry, who they believed had the potential to unseat Bush. 

Very active in the environmental, anti-war, and campaign-finance-reform movements, Gage was awarded the Council on Foundations' inaugural Robert W. Scrivner Award in 1986 for Creativity by an Individual Grantmaker.