Phil Mickelson's PGA Championship victory earns the bettor $300,000 in winnings.

Phil Mickelson (-6) won the 103rd PGA Championship at Kiawah Island, becoming the oldest major winner ever.

His two-shot victory over Louis Oosthuizen and Brooks Koepka was a mixed bag for sportsbooks, with one bettor winning $300,000 at DraftKings Sportsbook off a $1,000 bet on Mickelson to win at 300-1 odds. A few bets on long shots like Mickelson can quickly increase liability for books.

DraftKings sportsbook director Johnny Avello told VSiN, "Let's just say some of our customers will have a good Sunday."

Mickelson's odds were as high as 300-1 before the tournament, before falling to around 250-1 before Thursday's first round. Mickelson's odds at DraftKings were 90-1 before Round 2, 12-1 before Round 3, and +275 before Sunday's final round. By Sunday, Mickelson had the sixth-most bets (4.3 percent ) and the third-highest handle (6.5 percent ) at BetMGM. He was the tournament's biggest liability for BetMGM books heading into Sunday's final round; entering the tournament, 31 other players had more tickets to win than Mickelson.

A few notable bets were placed at William Hill sportsbooks, with one bettor in Nevada and one in Iowa wagering $100 each at 200-1 odds to win $20,000 each. “We were a good six-figure loser,” William Hill vice president of marketing Michael Grodsky told VSiN.

“It was about break-even for us,” said Jeff Sherman, vice president of risk at SuperBook Sports, via text message to VSiN. “Phil received some support over the weekend at 14-1 and 3-1, but only minor support at 250-1.”

Lefty's victory was even better for other books, according to Thomas Gable (sportsbook director at Borgata in New Jersey), who told VSiN that Mickelson's victory was a good result.

It was incredible to see the galleries and excitement on Sunday as Mickelson won another major, even if not all sportsbooks were happy to see him win.