Green party Leader Elizabeth May has appointed Coyne, a constitutional lawyer, to be her senior policy adviser.
Coyne's defection from the Liberals follows a series of unsuccessful attempts to become a Grit MP.
She ran for the Liberals in Toronto-Danforth against then-NDP leader Jack Layton in 2006, briefly ran for a nomination in another Toronto riding in 2008 before dropping out and lost a nomination bid late last year in an Ottawa riding.
Coyne also launched a long-shot bid for the Liberal leadership in 2013, in which she garnered less than one per cent of the vote; Trudeau won the contest in a landslide.
Coyne says in a statement that she now wants to help make the Green party the first choice for voters who are disillusioned with politics.
She says a vote for May in the coming federal election will be "a vote for principled national leadership."
ALSO ON HUFFPOST: A February 2013 interview with Deborah Coyne during her bid for Liberal party leadership
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