Speaking in Halifax on Wednesday, Trudeau said even if Mulcair wasn't the leader, there are too many policy differences to make a coalition possible.
"There's too many big issues on which the NDP and Liberal party have deep disagreements when it comes to economic growth, when it comes to trade, when it comes to the Constitution and how easy it should be to break up this country," he said.
"I do not see any possibility of coalition with the NDP regardless of who the leader might be."
During an interview with The Canadian Press on Tuesday, Trudeau was asked whether having someone other than Mulcair as leader would change the dynamic in terms of a coalition between the two parties.
Trudeau replied: "I don't know. ... Honestly, I don't want get into hypotheses. Maybe, but maybe not."
Mulcair reiterated his openness last month to a possible coalition with the Liberals if it is necessary to topple Stephen Harper's Conservatives. Some recent polls have suggested the possibility of a minority government in Ottawa after this fall's election.
Also on HuffPost