Lesnar revealed on Tuesday that he has re-signed with the WWE, ending specualtion that he would be making a return to the Octagon.
"I'm officially closing the door on MMA," Lesnar told ESPN. "It was hard. I re-signed last night with the WWE.
"It was a very hard decision at this stage of my career. The fighter inside me wants to compete. The father and husband — I'm an older caveman now. I make wiser caveman decisions. So, I'm here to say my legacy in the Octagon is over.
"It was a hard decision to make. It took me a year. I've thought about this for a year. At the end of the day, it's all about me wanting to have fun. The last two-and-a-half months I've been training to get back into the Octagon. I felt physically great, but something lacked mentally — and that's not good."
Last month, Lesnar was seen cageside during Ronda Rousey's women's bantamweight championship match against Cat Zingano at UFC 184 in Los Angeles.
However, Lesnar made it clear that he attended the event solely as a fan and did not go there with the intention of restarting his UFC career.
UFC CEO Lorenzo Fertitta confirmed this to ESPN.com earlier this week and added that he and Lesnar did not speak about a potential return.
"He hit me with a text two days before the fight and asked if he could have two tickets," Fertitta said. "It was kind of funny, that fight we had more requests for celebrities than we've ever had but we said, 'of course, we'll get you two tickets.' Then he called back and said he wanted three.
"He just wanted to come to the fight and I think feel it again, feel the energy. Remember what it was like to be part of the UFC."
Lesnar is set to compete in the main event of Wrestlemania 31 this weekend, where he will face Roman Reigns for the WWE World Heavyweight Championship.