Phil Mickelson has placed bets worth a total of more than $1 billion during his career, including a $400 000 wager on the outcome of the 2012 Ryder Cup, according to excerpts from a forthcoming book published in US media on Thursday.
The revelations about Mickelson's betting are contained in "Gambler: Secrets from a Life at Risk" by professional US gambler Billy Walters, a former close associate of the six-time major winner.
In an extended excerpt from the book published by the FirePit Collective website on Thursday, Walters detailed frenzied betting by Mickelson over the past three decades.
A biography of Mickelson published in 2022 by US golf writer Alan Shipnuck reported that the golfer had racked up more than $40 million in gambling losses throughout his career.
Mickelson last year acknowledged he had struggled with gambling in the past, describing his betting as "reckless and embarrassing" and revealing that he had undergone "hundreds of hours of therapy" to address the issue.
However Walters said the scale of Mickelson's losses had been significantly under-reported, and estimated the golfer had in fact made bets worth more than $1 billion in total.
"Based on our relationship and what I've since learned from others, Phil's gambling losses approached not $40 million as has been previously reported, but much closer to $100 million," Walters wrote.
"In all, he wagered a total of more than $1 billion during the past three decades."
Walters and Mickelson's relationship ruptured when Walters was investigated and subsequently convicted and jailed over an insider trading scam.
Mickelson wasn't charged, but repaid almost $1 million from money made in the deal.
Walters says in the book he never passed on inside information to Mickelson on the stock deal probed by authorities.
"All Phil had to do was publicly say it. He refused," Walters wrote.
"The outcome cost me my freedom, tens of millions of dollars and a heartbreak I still struggle with daily."
Walters meanwhile added that Mickelson had attempted to place a massive bet on the United States to win during the 2012 Ryder Cup, looking to wager $400 000 on an American victory.
Walters said he rebuffed the approach, comparing Mickelson to Pete Rose, the former baseball player and coach who was permanently banned from the sport in 1989 for betting on games he was involved in.
"I could not believe what I was hearing," Walters wrote, claiming he told Mickelson: "'Have you lost your ... mind?' I told him, 'Don't you remember what happened to Pete Rose?'"
"'You're seen as a modern-day Arnold Palmer,' I added. 'You'd risk all that for this?' I want no part of it."
Mickelson triggered uproar in world golf last year after becoming one of the earliest and highest-profile defectors to the Saudi Arabia-backed LIV Golf circuit in a lucrative deal reportedly worth $200 million.