The former cricketer beat American Richard Dawson on points after his fight of four two-minute rounds at the MEN Arena in Manchester tonight (November 30).
Flintoff managed to overcome being knocked to the canvas in the second round to take the bout 39-38, much to the delight of the 5,000-strong crowd.
"As a personal achievement, this tops the lot," he said after the contest. "It's not natural to me, I've had to work so hard.
"The feeling of being in there and winning at the end, I can't believe it, but I'm not pretending to be something I'm not."
He added: "The crowd made a massive difference tonight. I appreciate I was sloppy at times, but it was a humbling moment."
The ex-England all-rounder took part in the heavyweight showdown as part of the Sky1 series Flintoff: From Lord's to the Ring, which followed the 34-year-old in training for the match.
In preparation for the event, Flintoff had sparred for 300 rounds and lost three-and-a-half stone in weight over four months of training with Barry McGuigan and his son Shane.
He also received coaching from former World Heavyweight Champion Mike Tyson, which Flintoff described as "a pretty surreal experience".
Yesterday, he weighed in ahead of the fight at 216lbs (97.98kg), which was 25lbs (11.34kg) lighter than his opponent.
Flintoff: From Lord's to the Ring concludes on Thursday, December 6 at 9pm on Sky1.
Watch Digital Spy's video interview with Freddie Flintoff below: