A caucus member of the British Columbia Liberal Party, she served in several cabinet posts under premiers Gordon Campbell and Christy Clark.

[2][3] She worked in Toronto before moving with her husband Robert to Rossland, British Columbia in 1989, where she practised as a family physician.

[2][4] MacDiarmid ran as a candidate for the British Columbia Liberal Party in the October 2008 by-election for the provincial riding of Vancouver-Fairview, which took place as a result of incumbent MLA Gregor Robertson's decision to resign his seat in order to run for mayor of Vancouver at the 2008 municipal election.

The Ombudsperson recommended that by May 31, 2017, the government of British Columbia "make a public statement that acknowledges and apologizes for the harm caused by the Ministry of Health investigations and the decisions".

[18] Following her defeat in the 2013 provincial election by BC NDP candidate George Heyman,[19] she was appointed to the board of directors for Vancouver Coastal Health Authority, serving from 2013 to 2017.