Born in 1970, Mellish turned professional in 1991. In his first season, he enjoyed a run to the last 48 at the 1992 World Championship, defeating six opponents including Mike Dunn, Mike Darrington, Marcel Gauvreau, Les Dodd and Tony Chappel before being drawn against New Zealander Dene O'Kane. In their match, O'Kane compiled a century break in eliminating Mellish 10–4.