A longtime NDP supporter, MacDonald first attempted to enter provincial politics in the 1988 election, finishing third in the Cape Breton-The Lakes riding behind Liberal Bernie Boudreau and Progressive Conservative incumbent John Newell.[5] She ran again in the 1993 election, losing to Boudreau by more than 5200 votes, while edging out the Progressive Conservative candidate to finish second.[6]
Following Boudreau's resignation as MLA, MacDonald ran in the by-election to replace him.[7] On November 4, 1997, MacDonald won the by-election by 672 votes, becoming the first provincial New Democrat elected from Cape Breton since 1978.[8] MacDonald was also the first woman MLA elected in Cape Breton.[9] She was re-elected by 866 votes in the 1998 election,[10] and served as caucus chairwoman and gaming critic.[11] She was defeated when she ran for re-election in 1999, losing to Liberal Brian Boudreau by 101 votes.[12][13]
On March 22, 2000, MacDonald announced that she was entering the leadership election to replace Robert Chisholm as leader of the Nova Scotia New Democrats.[9][14] At the leadership convention on July 15, MacDonald trailed MLA Kevin Deveaux by four votes after the second ballot, but with the support of third place candidate Maureen MacDonald,[15] she overtook Deveaux on the third ballot to win the leadership.[2][16][17] MacDonald officially took over as leader on July 17.[18]
In October 2000, MacDonald announced that she would run in the Cape Breton North riding when a by-election was held to replace former Liberal premier Russell MacLellan.[19][20] On January 14, 2001, she was nominated to run as the NDP candidate for the riding.[21] On March 6, 2001, MacDonald finished third in the by-election, losing to Progressive Conservative Cecil Clarke.[22][23][24] Following the loss, MacDonald said she would continue to serve as leader, but resigned on April 24, after she learned that six members of the NDP caucus wanted to meet with her to ask for her resignation.[25][26][27] She was succeeded by Darrell Dexter.[28]