John Mugabi started as a professional on December 5, 1980, by knocking out Oemer Karadenis in round one in Cologne. Soon after that win, Mugabi moved to London where he became acquainted with boxing promoter Mickey Duff, an expert in boxer marketing who landed Mugabi various fights in England and built his reputation there. Mugabi won eight fights in Europe. Searching for more formidable foes, John moved to Florida where he became a favourite of American TV networks with his sensational knockouts of contenders such as Curtis Ramsey, Gary Guiden, former world champion Eddie Gazo, Curtis Parker, "Frank The Animal Fletcher", Nino Gonzalez and Earl Hargrove through a display of tenacity and ferociousness.'The Beast’ as he would become known (for his ferocious attacks and untamable style—bashing his prey with overhand rights and wild left hooks) was able to fluctuate his weight between middleweight and junior middleweight. Some people lose effectiveness by changing weight classes, but not Mugabi. He was able to knock out every opponent he faced to that point of his career.