Marion Bryden was elected to the Ontario legislature in the 1975 provincial election, defeating incumbent Progressive Conservative Thomas Wardle by 2,650 votes in the riding of Beaches—Woodbine. She faced Wardle again in the 1977 election and defeated him by 2,274 votes. In the 1981 provincial election, Bryden defeated Progressive Conservative Paul Christie by only 324 votes amid provincial setbacks for the NDP. She supported Ian Deans for party leader in 1978 which was won by Michael Cassidy. She supported Bob Rae for the party leadership in 1982.