Leslie will work alongside the party's foreign affairs critic, MP Marc Garneau, Trudeau said Wednesday.
Flanked by Trudeau and Garneau outside the Liberal caucus room in the House of Commons, Leslie spoke of his 35 years in uniform but said he hasn't yet decided whether he will seek a Liberal nomination to run in the next election.
The 'turning point" about joining the party came just a short time ago, he said.
"In response to the Quebec proposed charter," Leslie said, "the only national leader who stood up and articulated my views in a clear and convincing fashion was Justin, who said the proposed draft charter is not what Canadians want or need. It's divisive and discriminatory. That moment I knew he was a member of my team."
Asked if his addition to the Trudeau team was a sign the party was moving away from its traditional emphasis on peacekeeping, Leslie replied, "Sometimes to keep the peace you have to fight."
He said his presence wasn't a sign of a shift in the party at all. Liberals, he said, are "focused on the diplomatic, on peacekeeping, but are prepared to use force as they have throughout the sweep of time when it's absolutely required."
Leslie rose to the rank of lieutenant-general in the forces, and became chief of the land staff before he retired in 2011.
While in the army he wrote a controversial report about cutting costs in the military, recommending slicing the budget for contractors, consultants and private-sector providers by a third, and moving half the 9,000 full-time reservists back to part-time positions or having them enlist in the regular forces.
On Tuesday, Trudeau announced the formation of an economic advisory team headed by the new Liberal candidate for an upcoming Toronto byelection, Chrystia Freeland and finance critic Scott Brison.