Since the rapid growth of mixed martial arts during the mid 2000's, many have postulated on how a high level professional boxer would fare using MMA rules. Despite the ongoing debate however and rumors of several fighters considering crossing over, relatively few boxers have tried their hand.
For the time being, high level boxers still earn well in excess of all but the very biggest stars in MMA and as such entering the sport for them has little upside. In boxing the biggest stars can still more or less hand pick their opponents and as such make millions for relatively little effort. In MMA by contrast, champions are for the most part always made to fight the most deserving challenger, regardless of how much money the event will generate or how popular the fight will be with the fans.
Perhaps the biggest difference between boxers entering MMA and MMA fighters becoming boxers however is that the latter already box as part of their training. Certainly the stances and movements are vastly different between the two sports, however fighters still have a fundamental knowledge of striking. Boxers on the other hand in most cases have never wrestled, used kicks, elbows or knees and have no experience with jiu jitsu.
Despite the inherent difficulties in doing so however, several well known boxers have made the switch with varying levels of success.
Probably the highest profile boxer to cross over to MMA, however briefly as it turned out, was former multiple weight champion James Toney. Toney, if nothing else by that stage an inveterate showman, essentially talked his way into a UFC bout against MMA great Randy Couture, which he promptly lost. He has since returned to boxing and perhaps as a result of his short lived MMA fame has secured several relatively high profile fights.
Former heavyweight champion Ray Mercer similarly began competing in MMA following the end of his tenure as a top heavyweight boxer and amassed a record of 1-1 for his troubles. Losing to Kimbo Slice in 2007 before famously knocking out former UFC heavyweight champion Tim Sylvia in under ten seconds two years later.
Former two weight world champion Ricardo Mayorga is perhaps the best example of why top boxers will rarely cross over given the choice. Following a dispute with promoter Don King which had seen him inactive for more than a year, Mayorga signed a deal with MMA promotion Shine Fights to take on former UFC lightweight Din Thomas.
Shortly before the fight King decided to put a stop to things, filing for a breach of contract against Mayorga. Shine Fights and Mayorga maintained that King's contract pertained to boxing only, and therefore shouldn't control Mayorga's ability to compete in other sports. The presiding judge ruled in King's favor however, on the basis of a portion of his contract with King that stated he would not engage in any 'bouts' other than those promoted by Don King Promotions. The essential part being the absence of the word 'boxing' in the section, thus extending the contract to include bouts of any kind.
Shortly after the dispute, Mayorga settled his differences with King and was given a fight against then light middleweight champion Miguel Cotto for which he made a stated purse of $50k and likely a share of the pay per view money as well.
Even this very low purse for a fighter of Mayorga's caliber however was likely well in excess of what he would have made for his proposed MMA bout, even moreso if he did indeed get a share of the pay per view money.
Kevin Ferguson aka Kimbo Slice is something of a special case in that he has taken things a step further, having fought as a bare knuckle fighter, MMA fighter and professional boxer. In Slice's case however his motivations were seemingly financial and a question of ease.
Bare knuckle fighting and all the internet notoriety that came with it likely wasn't that well paid, not to mention being largely illegal and risky. MMA took Slice to bigger audiences and much larger paydays although his fundamental lack of well roundedness as a fighter and advanced age quickly saw holes in his game becoming apparent and his drawing ability dwindle. Becoming a boxer has seemingly worked out well so far for Slice, although of course he is seemingly being brought along very slowly against meagre opposition for the time being.
Numerous other fighters have supposedly mooted taking part in MMA bouts at some point in the future including Miguel Cotto, David Haye, Floyd Mayweather, Vic Darchinyan, and several others although as yet none have actually made the switch. Time will tell whether any of those mentioned above will genuinely become the first high level boxer to cross over although given their ages, (all are well into their thirties) any MMA bouts would likely be well past their best days as boxers.