Born and raised in Louisville, Kentucky, and honed his game at Harmony Landing Country Club in nearby Goshen, Kentucky, where his father, Mike, served as head professional for three decades (1990 through 2020) and later became Golf Professional Emeritus. Justin's mother, Jani, ran the pro shop at Harmony Landing.

One of his favorite golf memories is making a hole-in-one with a cut-down driver on the 144-yard par-3 16th at Harmony Landing Country Club when he was six years old. Thomas noted, "For years it was the only hole I could reach in regulation. My dad had made an ace there, and so had my grandpa, Paul. Three generations scoring aces on the same hole. How many families can say that?"

Prepped at St. Xavier High School, a powerhouse golf program in Louisville, Kentucky. As a junior, Thomas won the individual state title at the 2009 Kentucky High School Athletic Association Boys State Championship, and helped St. Xavier win back-to-back team state titles in 2008 and 2009 (the 17th and 18th team state titles in program history). Other notable St. Xavier alums include 12-time PGA TOUR winner Bobby Nichols, who won individual state titles in 1953 and 1954, and 11-time PGA TOUR winner Frank Beard, who won the 1957 individual state title.

Son of Mike, a retired PGA professional, and Jani Thomas. Became the eighth son of a PGA professional to win the PGA Championship when he won in 2017. Late grandfather, Paul, was a PGA professional for 60 years.

Host of the American Junior Golf Association's Justin Thomas Junior Championship, established in 2016 and held annually in Kentucky. Won three AJGA titles as a junior.

Appeared on the cover of the video game "PGA TOUR 2K21," released August 21, 2020.

Announced a partnership with Jack Nicklaus in 2022 to design Panther National, a golf course in Palm Beach County, Florida.

Made a cameo appearance in Happy Gilmore 2, released on Netflix in July 2025.

Played two seasons at University of Alabama (2011-13), where he broke school records for career scoring average (70.73), career wins (six), single-season scoring average (70.44 in 2011-12), and single-season wins (four in 2011-12). Notably earned individual medalist honors at the 2012 Southeastern Conference Championship.

As a freshman at Alabama, earned the 2012 Jack Nicklaus Award and 2012 Haskins Award -- two of the three national player of the year awards for collegiate golf. Also earned the 2012 Phil Mickelson Outstanding Freshman Award.

Alongside fellow future PGA TOUR winner Trey Mullinax, helped Alabama win its first national title at the 2013 NCAA Championship. Alabama defeated a University of Illinois team led by future PGA TOUR winners Brian Campbell and Thomas Detry in the championship match, though Thomas lost his match against Thomas Pieters.

As a 16-year-old amateur, became the third-youngest player to make the cut in a PGA TOUR event at the 2009 Wyndham Championship. Earned the sponsor exemption for his PGA TOUR debut as the winner of the 2009 FootJoy Invitational, an AJGA event contested at Sedgefield Country Club.

Runner-up finisher at the 2010 U.S. Junior Amateur Championship. Notably defeated Denny McCarthy in the semifinals, but fell to Jim Liu in the championship match.

Qualified for match play at three consecutive U.S. Amateur Championships (2010, 2011, 2012), with his best finish being a semifinals appearance at Cherry Hills Country Club in 2012. Path to the semifinals in 2012 included victories over Max Homa (Round of 32) and Bobby Wyatt (Round of 16).

Represented the United States at the 2012 and 2013 Palmer Cups, 2012 World Amateur Team Championship (won Eisenhower Trophy), and 2013 Walker Cup, his final event as an amateur. Teammates on the victorious Walker Cup team included Max Homa, Michael Kim, and Patrick Rodgers. Teammates on the victorious 2013 Palmer Cup team included Kim, Rodgers, and Daniel Berger.

One of three recipients of the Byron Nelson International Junior Golf Award in 2011, with the others being Denny McCarthy and Michael Johnson. The award recognizes the achievements of junior golfers who distinguish themselves with a high level of junior tournament play, as well as excellence in academics, exceptional character, and a commitment to community service.