Justin Masterson was born in Kingston, Jamaica, where his father served as dean of students at the Jamaica Theological Seminary. A few years later, Masterson moved to Fort Wayne, Indiana. He attended Beavercreek High School in Beavercreek, OH where he first played baseball as a catcher, pitcher, and first baseman.

Masterson attended Bethel College in Mishawaka, IN, where he played as a starting pitcher. While at Bethel, he hit 10 home runs during his sophomore year. He then attended San Diego State University. As a relief pitcher, Masterson pitched out of the bullpen in the Cape Cod Baseball League in 2005, saving 10 games for Wareham while posting a 1.15 ERA.

Masterson pitches at a low three-quarter arm delivery with a sliding motion, which some compare to Dennis Eckersley's delivery. His pitching arsenal includes a fastball that reaches 97 mph, a sinker, a slider, and an occasional change-up. It has been claimed that his best pitch is his plus-sinker with a heavy drop.

Masterson throws a variety of fastballs that vary in speed and break, with the speeds reaching between from the upper 80s to the mid 90s. The speed of his slider is in the low 80s, and his change-up ranges from 77 to 81 mph. He can vary his sinker at speeds of 84 to 96 mph, sometimes catching batters off balance.

Masterson was signed to the minors by Dan Madsen, and was both a starter and reliever in his minor league career for the Lowell Spinners. In 2007, his second season, Masterson was promoted from the Class A Lancaster JetHawks to the Double-A Portland Sea Dogs. Masterson's 12 wins that season ranked second among all Red Sox minor leaguers.

In 2006, Masterson was named to the Baseball America short season all-star team. Masterson was converted to starting pitcher in early 2007, after a stint as a relief pitcher for Short Season Lowell in 2006.

The Red Sox invited Masterson to spring training during early 2008. On April 24, 2008, he made his Major League debut against the Los Angeles Angels of Anaheim in an emergency call-up start. He pitched six full innings and allowed one run. On May 20, 2008, Masterson made his second appearance, pitching 6.1 innings, allowing three hits and one run with three walks and five strikeouts and picking up his first win in the Majors.

Masterson picked up the first postseason win of his career in Game Five of the 2008 American League Championship Series against the Tampa Bay Rays, pitching a scoreless ninth inning in the Red Sox' 8–7 victory.

On July 31, 2009, Masterson was traded along with minor league prospects Nick Hagadone and Bryan Price to the Cleveland Indians for Victor Martinez.

Masterson pitched his first professional complete game shutout on June 9, 2010 against the Red Sox, his former team, pitching a two-hitter.

Upon the conclusion of the 2011 season, a season in which Masterson went 12-10 with a 3.21 ERA and 216 IP, he underwent surgery to repair a torn labrum in his non-throwing shoulder.

On April 5, 2012, Masterson was the Opening Day pitcher for the Indians against the Toronto Blue Jays in what proved to be the longest Opening Day game in major league history, a 7-4 loss in 16 innings. Masterson threw 8 innings, giving up 2 hits and 1 run while striking out 10 and earned a no-decision.

On April 2, 2013 Masterson became the back-to-back Opening Day starting pitcher for the Indians. He defeated the Toronto Blue Jays 2012 Cy Young winner, R.A. Dickey. Masterson's second win was against David Price, Rays 2012 Cy Young winner. According to Elias Sports Bureau, Masterson became just the third player in MLB history to beat both reigning Cy Young winners in the same season -- and the first to do in his first two starts of the season.

Masterson is a Christian and often speaks of the importance of God in his life. He and his wife are active in philanthropy, supporting organizations like Bright Hope and Not For Sale Team.

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