Mercer turned pro in January 1989 and debuted with a 3rd TKO of Jesse Hughes. He scored a series of knockouts and in August 1990 knocked down and outpointed big punching Smokin' Bert Cooper in a spectacular 12 round brawl that earned him Cooper's NABF title. In January 1991 he challenged undefeated Francesco Damiani for the WBO heavyweight title, scoring a one punch knockout victory in the 9th when behind on points. Later that year he beat undefeated puncher Tommy Morrison in five, and with a major world title fight on the horizon vacated his WBO belt and fought 42-year-old legend Larry Holmes rather than mandatory challenger Michael Moorer. It proved an unwise decision, as the crafty Holmes conned Mercer out of the fight, outjabbing the puzzled Mercer and gaining both the points decision, and Mercer's world title fight with heavyweight king Evander Holyfield.

This Everlast boxing glove was signed in black by Mercer and includes a JSA sticker and COA.