Lynn was born on May 12, 1987, in Indianapolis, Indiana. He graduated from Brownsburg High School, where he was teammate of fellow major league pitcher Drew Storen. His team went 33–0 his senior year and won the 4A state championship. He was drafted by the Seattle Mariners in the 6th round of the June 2005 Major League Baseball Draft, but did not sign. He played at the University of Mississippi. In 2007, he set the Ole Miss single season (146) and career (332) strikeout records. He was named All-Southeastern Conference 2nd-team pitcher in 2007 and 2008. He is 6' 5", 250 lbs. He married Lauren (Grill) Lynn in November 2010, who was a softball player at Ole Miss. Their home base is near Wilsonville, Oregon. On MLB's Now You Know he stated that if he were not a professional baseball player he would be a UPS man.
Lynn was drafted by the St. Louis Cardinals in the 1st round (39th) of the 2008 MLB June Amateur Draft. In 2009, Lynn was named the Cardinals organizational Pitcher-of-the-Year after jumping three levels in his first full professional season. On September 10, 2010, Lynn broke the Memphis Redbirds franchise strikeout record, fanning 16 against the Oklahoma City RedHawks.
Lynn was activated on June 2, 2011 and made his major league debut that night at home against the San Francisco Giants. On October 10, 2011, Lynn was awarded the win after throwing exactly one pitch in Game 2 of the 2011 NLCS. In the 2011 World Series, Lynn gave up back-to-back home runs in the top of the 7th to Adrián Beltré and Nelson Cruz. He won the 2011 World Series with the Cardinals, making 10 appearances during the postseason. He only appeared in 18 games during the 2011 regular season.
Lynn announced he would begin wearing the number 31 after arriving at Cardinals spring training in February 2012. In 2012, he was promoted to the role of starting pitcher, replacing the injured Chris Carpenter in the rotation. Lynn started strong in 2012, winning his first six games with an ERA of .140. One of his best games came on June 13, against the Chicago White Sox. He went 7 ⅓ innings, struck out a career-high 12 batters, and gave up only 3 hits. He also became the second player that season to win 10 games behind RA Dickey. He was later removed from the starting rotation in August after Carpenter's return, but added back after Jaime Garcia was added to the disabled list due to a rotator cuff injury in the NLDS.
Lynn throws four pitches: a four-seam fastball (93–95 mph, tops out at 97 mph), a two-seam fastball (92–93), a cut fastball (87–88) and a curveball (79–80). He uses his cutter mostly against right-handed hitters. Besides his four-seam fastball, his curve is his most commonly thrown pitch with two strikes. His four-seamer has one of the highest whiff rates among fastballs thrown by relief pitchers. He has been a good strikeout pitcher, finishing third in the National League in 2012 in K/9, at a 9.2 rate. His 33 wins between 2012-2013 were the most by any NL starter over that span.