In June 2015, he became the first SMU Mustang to win the NCAA individual championship, recording a score of 280 (−8) to win by one stroke. In August, he won the U.S. Amateur title, defeating Derek Bard 7 & 6 in the 36-hole final. He became the fifth player to win both the NCAA and U.S. Amateur titles in the same year, joining Jack Nicklaus (1961), Phil Mickelson (1990), Tiger Woods (1996), and Ryan Moore (2004).
Lowry made his professional debut in the 2009–10 UEFA Europa League in an away fixture at Rapid Vienna. On 17 September 2009, Lowry signed a three-month loan deal with Football League Championship side Plymouth Argyle. Lowry made his Plymouth Argyle debut on 19 September 2009, playing 90 minutes, in a 3–1 loss against Newcastle United. He played 13 league games for Plymouth during his loan spell, where he impressed many and was recalled by Villa who required Lowry's services as cover after some injuries to defenders.
U.S. Open: Established the first-round lead at the U.S. Open with a 7-under 65 and was in the mix all week at Erin Hills. Followed with 73 to be one back through 36 holes. Third-round 68 had him two adrift headed into final round. Closed with even-par 72 to finish six back of Brooks Koepka at T5. In nine U.S. Open appearances, has advanced to the weekend five times and never broken par in the final round.
While serving a suspension, Lowry signed a two-and-a-half-year permanent deal to Millwall for an undisclosed fee on 27 January 2012. Lowry then made his first appearance for the club since joining Millwall permanently, playing 90 minutes, in a 2–1 loss against rivals' West Ham United on 4 February 2012. Right way through March, Lowry was sidelined with ankle injury that kept him out for weeks. After making his first team return against Cardiff City, in a 0–0 draw on 31 March 2012, Lowry scored his first Millwall goal, which he scored the only goal in the game, in a 1–0 win over Coventry City on 17 March 2012. Lowry finished his 2011–12 season, making twenty-two appearance and scoring once.
What this looks like to the naked eye is a DeChambeau who is visibly larger than he was two months ago, when he first teased the idea of a bigger, stronger self. He was under 200 pounds then — now he’s 222. “I’m going to become like a gymnast. I watch online, on Instagram, these gymnast influencers, and that’s where I want to get.” It’s a step in the right direction, but 222 is not nearly big enough. DeChambeau has a Hulk-like vision for his future.
Enjoyed a stellar rookie season, with seven top-10 finishes and a 32nd-place finish in the FedExCup standings that led to him winning PGA TOUR Rookie of the Year honors. Finished inside the top 30 on the money list to qualify for the Masters and the U.S. Open. Played in first three Playoff events, slipping from 21st to 32nd and out of the TOUR Championship.
OHL Classic at Mayakoba: Birdied two of his last three holes at the OHL Classic at Mayakoba to fire a final-round 67 and finish in 2nd place, one shot back of Patton Kizzire. In his first start at the event, opened with a bogey-free, 6-under 65 to sit three back of Kizzire's lead. Followed with three-consecutives rounds of 67 in rounds two through four, resulting in a score of 18-under 266. Due to inclement weather Saturday, played his final two rounds on Sunday.
Lowry elected to turn professional the week after his Irish Open triumph, meaning that he would miss out on the opportunity to play in the Walker Cup in September 2009. He made his professional début on 28 May at the European Open, where he shot 78 in the first round to end the opening day almost at the back of the field, and went on to miss the cut following a second round 73.