Medlen spent the entire 2008 season with the Mississippi Braves of the Southern League. He made 19 relief appearances from April 5 to June 1, and got his first start on June 5, 2012, against the Mobile BayBears (an Arizona Diamondbacks affiliate). He recorded 10 strike-outs in two of his starts, in a 7.0 inning start against the Pensacola Blue Wahoos (on July 7, 2008), a game in which he allowed 4 hits and 1 walk, but also allowed 3 runs, and in a 6.0 inning start against the Carolina Mudcats (on August 22, 2008), a game in which he allowed 3 hits and 1 walk, but no runs. For the season, Medlen started 17 games, with 6 wins and 5 losses.
Medlen began the 2012 major league season as a reliever for the Atlanta Braves, typically pitching middle innings before the Braves set-up man and closer came on for the 8th and 9th innings. He was sent down to the Braves AAA team (the Gwinnett Braves), for the purpose of "stretching him out" to return to the majors as a starter. He started only three games for the Gwinnett Braves, pitching 2.1, 5.0 and 6.0 innings and having a record of two losses and one "no decision". He returned to Atlanta on June 18, 2012, and made 16 appearances in relief, from June 18, 2012 through July 25, 2012.
Medlen got his first start of the 2012 season, on July 31, 2012, against the Miami Marlins, allowing 4 hits and 1 earned run and recording the win in a 7–1 game. After an August 5 "no decision" against the Houston Astros (which the Braves won by a 6–1 score), Medlen won five consecutive games in dominating style. On August 11, 2012, in a 9–3 win against the Mets at Citi Field, Medlen pitched 6.1 innings, allowing 5 hits and 1 walk, while striking out 7.
Medlen recorded his first complete game shut-out on August 16, 2012, with a 6–0 victory over the San Diego Padres, in which allowed 5 hits and struck out 6, allowing no walks. Perhaps the most important win of Medlen's professional career came on August 22, at Washington, with the Braves ending a four-game losing streak that had included two games against the Division-leading Nationals. Medlen pitched 7 innings, allowing 7 hits and 1 walk, while striking out 7. The win allowed the Braves to remain six games behind the Nationals. With the win over the Nationals, Medlen also broke a franchise record with the Braves winning the 16th consecutive game that Medlen had started. Medlen's fourth win in the month of August 2012 came at San Diego, on August 28, with Medlen pitching 8 shut-out innings. Medlen allowed 5 singles, but picked off two of these runners, and no runner moved beyond first base. He struck out 9 Padres, and allowed no walks. Once again, the win by Medlen allowed the Braves to avoid being swept in a three-game series. With this game, Medlen extended his streak of scoreless innings to 28-1/3.
On September 3, 2012, Medlen faced the Colorado Rockies at Turner Field, and got the win in dominant fashion. Medlen pitched a complete game, allowing five hits and no walks while striking out a career-best 12 batters. He threw 111 pitches, with 88 of them being strikes. Medlen's streak of consecutive scoreless innings came to an end (at 34-2/3 innings) in the 7th inning when Carlos Gonzalez scored on an error by Paul Janish. This was the longest streak of consecutive scoreless innings for a Braves pitcher since Greg Maddux in 2000.
On September 8, 2012, Medlen faced the New York Mets at Citi Field, and recorded his 8th win of the season. Medlen had completed 6 innings, allowing 4 hits and 2 earned runs, when the game was delayed because of rain. Medlen did not return after the 75-minute delay, but got credit for the win in the 11–3 victory for the Braves. When he allowed an earned run in the 4th inning, his streak of consecutive innings without an earned run ended at 40-1/3 innings. This streak had started at Citi Field on August 11. His streak of innings without allowing a home run ended at 53 innings, when Cedeno hit a home run in the 5th innings.
On September 14, 2012, Medlen got the start against the first-place Washington National, at Turner Field, and pitched 7.0 innings, leaving with the score tied at 1–1. In his 7 innings, Medlen allowed 5 hits and 1 walk, while recording 13 strike-outs, a career high. The Braves went on to win the game 2–1, with a run in the bottom of the ninth, winning their 20th consecutive game that Medlen had started. This 20 game streak was a franchise record for the Braves and tied a major record set by the New York Yankees and Roger Clemens in 2001. Medlen had 8 called third strikes in the game, the most of any major league pitcher in 2012 and the most for any Braves pitcher since Greg Maddux in 2000.
On September 19, 2012, in Atlanta, Medlen pitched 8 scoreless innings, in a game that Atlanta won, 3–0. Medlen allowed 4 hits and 1 walk, while striking out 6 Marlins. In the 1st and 5th innings, Marlin runners reached third base with only one out, but a strike-out followed by a ground-out and a pop-out followed by a grounder preserved the scoreless game for Medlen. With this win, the Braves had won 21 consecutive games started by Medlen, the most in major league baseball since the Yankees won 22 consecutive games started by Whitey Ford from 1950 through 1953, a span interrupted by his service during the Korean War. On Sep 30, 2012, Medlen led the Braves to their 23rd consecutive victory in games that he started, as they beat the Mets, 6–2. Medlen pitched 6 Innings, allowing 3 hits, 1 walk and 1 unearned run, while striking out 4. This established a new major league record in that category, according to mlb.com.
On September 4, 2012, Medlen was named National League "Pitcher of the Month" for August 2012. In an article discussing the award, Atlanta Journal-Constitution writer David O'Brien pointed out that many commentators have been comparing Kris Medlen to the Braves' legendary pitcher, Greg Maddux. Medlen repeated as National League "Pitcher of the Month" for September, becoming the first Braves pitcher to win this back-to-back since Greg Maddux, in June and July 2001.
Medlen's highly effective changeup has a career whiff rate of 44%, among the highest rates for starting pitchers in the big leagues. Hitters in the 2012 season hit only .098 against the pitch.
Medlen met his wife Nicki through former minor league teammate Ryne Reynoso, and married her in 2012. They have two children.
