"We feel better about our pitching today than we did yesterday," Brewers general manager Doug Melvin said.
Marcum started for the Blue Jays on opening day last season and went 13-8 with a 3.64 ERA, a year after missing the entire 2009 season following Tommy John surgery. He turns 29 next week.
Marcum was especially attractive to the Brewers because he comes with a relatively low salary -- he made $850,000 last season -- and isn't eligible for free agency for two more years.
The Blue Jays have been stockpiling young talent in recent years, and there was talk they might try to put together a trade package to go after former AL Cy Young Award winner Zack Greinke of Kansas City.
"We feel we have some depth with our starting pitching," Toronto GM Alex Anthopoulos said. "Contrary to the Brewers, we're lacking in position players."
Lawrie is a 20-year-old second baseman from Canada and was rated among Milwaukee's top minor leaguers. He hit .285 with eight home runs, 63 RBIs and 30 stolen bases in Double-A last season.
Lawrie made the Canadian Olympic team in 2008 as a teenager and the country's 2009 club for the World Baseball Classic. His path to the majors with Milwaukee, at least at second base, was blocked by Rickie Weeks.
Anthopoulos said he'd be interested in acquiring Lawrie for quite some time. The GM said there was a chance Lawrie might someday be switched to third base.
This was the second trade between Toronto and Milwaukee in a matter of days. The Brewers sent reliever Carlos Villanueva to the Blue Jays last Friday.