UK
- Mark Williams (actor) (born 1959), British actor and comedian
- Mark Williams (politician) (born 1966), British Member of Parliament for Ceredigion
- Mark Williams (Holby City), fictional character (British drama series Holby City)
- Mark Williams (Northern Ireland footballer) (born 1970), Northern Irish footballer
- Mark Williams (snooker player) (born 1975), Welsh professional snooker player
- Mark Williams (footballer born 1982), former English footballer who played as a midfielder
- J. Mark G. Williams, academic and author specializing in depression and suicide
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