He played six seasons in Major League Baseball (MLB) for the Miami Marlins and Toronto Blue Jays.

After not appearing in a major league game since 2017, Koehler retired in 2020 due to numerous injuries, and became a MLBA certified baseball agent with Pro Agents Inc. Koehler ranks top ten in numerous Marlins franchise pitching statistics, including games started, innings pitched, strikeouts, earned run average (ERA) and walks plus hits per inning pitched (WHIP).

[3] During his senior season, Koehler went 6–1 with a 0.83 earned run average (ERA) and 108 strikeouts in 59 innings pitched.

[8] He was assigned to the Class-A Short-Season Jamestown Jammers of the New York–Penn League to start his professional career.

[10] During the 2009 season, Koehler played with the Class-A Greensboro Grasshoppers, and the Class A-Advanced Jupiter Hammerheads.

[10] He then received a promotion to the Jupiter Hammerheads of the Florida State League, where he went 4–1 with a 3.38 ERA and 25 strikeouts in 6 starts.

[10] Between the two clubs that year, Koehler earned a combined record of 9–6 with a 3.25 ERA and 107 strikeouts in 133 innings pitched.

[8] He made his first MLB start in the last game of the season against the New York Mets, in which he went five innings, allowing three earned runs and striking out three batters in a 4–2 Marlins loss.

[16][17] Koehler began the 2013 season with the Zephyrs and became the franchise's all-time strikeout leader on April 14 when he picked up his 267th against the Round Rock Express.

[20] He earned his first major league win against the San Francisco Giants on June 20, pitching seven innings and allowing only one run.

[8] On August 19, 2017, the Marlins traded Koehler to the Toronto Blue Jays for minor league pitcher Osman Gutierrez.

[22] He made his first start for the Blue Jays on August 24, pitching five innings, allowing one run, and striking out seven batters in a 2–0 loss to the Tampa Bay Rays.

[24] On December 20, 2017, Koehler signed a one-year, $2 million, incentive laden contract with the Los Angeles Dodgers.