Ryan Braun hit his first home run of the year for the Brewers, a solo shot with the game well out of reach in the ninth.
It was the third career start for Lynn (1-0), who played a key relief role during the Cardinals' championship run last season. The 24-year-old right-hander was added to the rotation after Chris Carpenter went on the disabled list with a shoulder problem this spring.
Corey Hart had a solo homer in the fifth for Milwaukee after hitting a pair of home runs Saturday.
Link to Boxscore: St. Louis 9 at Milwaukee 3