"As a lawyer, university professor, constitutional activist, public servant and writer, Deborah’s skills and hard work have placed her inside the great public debates of our times,” May said in a press release Thursday.
"Deborah’s thoughtful approach... complements the vision of the Green Party of Canada," the Green Party leader said.
Coyne was expected to run for the federal Liberals following her failed leadership bid. She placed fifth in the 2013 leadership race, with fewer than one per cent of the votes.
In the 2006 federal election, she ran unsuccessfully as a Liberal against the late NDP leader Jack Layton in the Toronto-Danforth riding. She filed nomination papers in a different Toronto riding in 2008 but later withdrew from the race.
The constitutional lawyer was said to be interested in an Ottawa riding for 2015, but did not secure a nomination.
Coyne has a family connection to Liberal Leader Justin Trudeau, whose father, the late Pierre Trudeau, is also the father of Coyne's daughter, Sarah. However, Coyne has written that Trudeau's two families were "separate" and she barely knew Justin Trudeau during her 15-year relationship with his father.
Coyne is also a cousin of political columnist and CBC At Issue panellist Andrew Coyne.