[5] Gainey studied Political Science at McGill University, before completing a Master's Degree in International Relations at the London School of Economics.

Following graduation, Gainey worked as a policy advisor to the Ministers of National Defence and Veterans Affairs, Bill Graham and John McCallum.

[6] During this time, she became a close friend of future Canadian Prime Minister and fellow Montrealer Justin Trudeau, actively advising on his campaign to become leader of the Liberal Party of Canada.

Following a break from federal politics, Gainey was briefly Chief Executive of the Policy think-tank Canada 2020, the advisory board of which is chaired by economist Mark Carney.

[8] In spring 2023, Gainey successfully ran for the Liberal nomination in the federal riding of Notre-Dame-de-Grâce—Westmount, beating Vice-President and general counsel of Air Canada, Fred Headon.