The American joined Tiger Woods, Jack Nicklaus, Jordan Spieth and Rory McIlroy in the select group of players who won two majors before turning 25.
The last two U.S. Open winners are both 1 over par after the first round at Royal St. George's, along with defending champion Shane Lowry. Thomas is 2 over par.
Louis Oosthuizen leads the way at Royal St. George, with Jordan Spieth and Brian Harman a shot further back.
World No. 1 Jon Rahm is the bookmakers' favorite as he looks to win a second consecutive major.
Security quickly escorted the thief, who also tried to get away with a dog-shaped head cover, off the course for nabbing the golf star's club.
The Spaniard won his maiden major after shooting 4-under par in the final round at Torrey Pines on Sunday.
Russell Henley and Louis Oosthuizen lead the way at Torrey Pines, with Brooks Koepka and Jon Rahm two shots adrift.
The third major of the season gets underway at Torrey Pines on Thursday with Jon Rahm as the bookmakers' favorite.
Corey Conners is the bookmakers' favorite at 7-1 after taking a two-shot lead in the first round at Kiawah Island.
Canada's Corey Conners leads by two strokes after shooting a 5-under par 67 in the first round, with Brooks Koepka and Phil Mickelson two and three shots behind.
Rory McIlroy is the bookmakers' favorite to win the second major of the season, followed by Jordan Spieth and Jon Rahm.
The second major of the season gets underway on Thursday at Kiawah Island Golf Resort in South Carolina.
Dustin Johnson missed the weekend cut, but he'll be there Sunday to don the green jacket on this year's champion. Justin Rose leads after two days and Jordan Spieth is charging.
Woods, who moved to a new hospital on Friday, said the golfing community was "truly helping me get through this tough time."
First-round leader Justin Thomas tees off at 1:27 p.m. ET alongside 15-time major winner Tiger Woods.
Justin Thomas leads the U.S. Open by one shot after carding a 5-under 65 in the first round on Thursday.
Tiger Woods starts the first round of the U.S. Open at 8:07 a.m. ET on Thursday from tee No. 1 alongside Justin Thomas and Collin Morikawa.
The first round of the U.S. Open gets underway at 6:50 a.m. ET on September 17, with world No.1 Dustin Johnson set to hit his first tee shot shortly after 1 p.m. ET.
Patrick Reed and Jason Day may both be dark horses at Winged Foot, and could Phil Mickelson finally complete a career Grand Slam?
World No. 1 Dustin Johnson is the man to beat in the second major of the season, with No. 2 Jon Rahm and No. 7 Xander Schauffele among his leading rivals.
Dustin Johnson begins the final round of the PGA Tour season finale with a five-shot lead over Justin Thomas and Xander Schauffele.
World No.1 and FedEx Cup leader Dustin Johnson arrives at East Lake as the man to beat this weekend.
FedEx Cup leader Dustin Johnson will start the season finale at 10-under, two shots ahead of world No.2 Jon Rahm.
Tiger Woods will begin the second round at 4:58 p.m. ET alongside world No. 1 Justin Thomas and Rory McIlroy.
Brooks Koepka trails the leaders by one shot after the first round, while Tiger Woods is two shots further behind in his second start since February.
The 15-time major winner has been paired with world No. 1 Justin Thomas and four-time major winner Rory McIlroy for the first two rounds.
The first major of the season begins at TPC Harding Park in San Francisco on Thursday as Brooks Koepka looks to win a third consecutive title.