Lesnar announced he had re-signed with the WWE in an interview on ESPN's "SportsCenter."
Moments after his announcement, Lesnar told The Associated Press he was in training for a UFC return as recently as last week, even though he had developed conflicted feelings about it over the previous month.
He finally decided to stick with professional wrestling only in the past two days, signing his new three-year deal with the WWE on Monday night when the promotion increased its financial offer.
"The fighter in me wants to continue, but at this stage in my life, it ain't just about me anymore," Lesnar told The AP. "You put your pride to the side. You hug your wife and your kids. I'm a 37-year-old man, and some days I feel like I'm 80, just with all the things I've experienced, all the things I've done. I feel fortunate about it. It's like, what else can I do? Why go backward?"
More than a year after Lesnar first began working on an MMA comeback, he decided his long-term health was more important than his hunger to return to competitive sports.
The UFC didn't allow Lesnar to shut the octagon door easily: Lesnar said he turned down an offer worth "10 times" what he was making earlier in his MMA career.
UFC Chairman Lorenzo Fertitta even attempted to increase the temptation when Lesnar phoned Fertitta and UFC President Dana White to tell them his decision while driving into the parking garage at ESPN's downtown Los Angeles studios.
Although the UFC doesn't disclose the financial results of its pay-per-view shows, White has said Lesnar is the biggest PPV draw in MMA history.
"Lorenzo said to me, 'Can we sharpen our pencil? Can we double it'" Lesnar said. "I said, 'It's not about that. I'm calling you to tell you where my heart is, and it's not about the money.'
"And then in the back of my mind, I'm thinking, 'Well, yeah, it is about the money, but I don't have to beat myself up for it.' To prepare for another MMA fight, we're talking 16, 18 weeks of pure hell, and then the cage door shuts and it's on."
Lesnar lost his title belt to Cain Velasquez in October 2010, and had major surgery several months later to remove a 12-inch section of his colon damaged by diverticulitis. He hasn't fought in the UFC since December 2011, when he lost to Alistair Overeem.
Persistent health problems surrounding his diverticulitis forced the 6-foot-3 Lesnar to take a new perspective on his athletic invulnerability. The hulking NCAA champion wrestler who nearly made the Minnesota Vikings' roster out of his first NFL training camp despite no real football experience had to re-evaluate his own mortality and ability.
"When you're sick for two or three years and you don't know what's going on, all of a sudden I went from the baddest man on the planet, to vulnerable," Lesnar said. "It's reality. My whole life I've been this superhuman freak that just kills people, a savage beast. I wasn't that guy anymore. ... Of course my confidence was totally jaded on my last three fights. Whose wouldn't be? Is Anderson Silva the same guy he was (after breaking his leg)?
"I've been a barbarian my whole life. I'm just a smarter barbarian now. Evolution, you know?"
But Lesnar still gathered his fight camp at his home in Minnesota in recent months to prepare for a big-money UFC return. He said his training has been going splendidly, with his coaches "absolutely amazed" by his progress.
Yet doubt kept clawing at Lesnar's mind. He had a sleepless night a month ago, when he began talking audibly to himself about a return. He booked his own ticket to Los Angeles last month to attend the UFC 184 card highlighted by Ronda Rousey's latest dramatic victory, hoping the energy and atmosphere of a big-time title fight would rekindle his fire.
"I tried to picture myself coming down to the cage, and it was like a bad dream," Lesnar said. "It just wasn't right. It didn't feel right."
Lesnar realizes his athletic retirement is cushioned by his busy schedule with the WWE, and he still gets a measure of competitive thrill from pro wrestling's scripted results. He also will have more time to spend with his wife and three children, and Lesnar seems confident he can thrive without MMA.
"I'm riding the caboose of my sports career," Lesnar said, waving his hand in the air. "I just barely caught this thing. I'll see you the next time."