The two political veterans and Vancouver lawyer and professor Brian Fixter had been competing for the opportunity to run for the seat currently held Liberal MLA Mary McNeil, who is not running again.
The riding association election was held Wednesday night, giving Sullivan 273 of 517 votes.
Mayencourt was hoping to return to the job he quit before the 2009 provincial election.
He was replaced by McNeil after stepping down to run for the federal Conservatives in an unsuccessful bid to unseat Vancouver-Centre Liberal MP Hedy Fry.
Sullivan, who was mayor from 2005 to 2008, will face the B.C. NDP’s Matt Toner and the B.C. Green Party’s Daniel Tseghay in the May 14 election.