McInerney was at the far post to bang in Nigel Reo-Coker's cross.
The second leg is May 13 in Toronto.
The Canadian Championship winner will play in the 2016-17 CONCACAF Champions League. The Impact advanced to the final this year, losing to Club America of Mexico last week.
The start of the other semifinal between the Vancouver Whitecaps and FC Edmonton was postponed Wednesday because of unsafe conditions following snow in Edmonton. The first leg will be played in Vancouver on May 13, and the second in Edmonton on May 20.