Thomas McMahon (born 17 June 1936, in Dorking, Surrey) is an English Roman Catholic bishop.

McMahon grew up in Harlow and attended St. Bede's Grammar School, Manchester, before training for the priesthood at St. Sulpice, Paris.

On 16 June 1980, Pope John Paul II appointed Fr McMahon as the Bishop of Brentwood.

He was a founder member of the Movement for Christian Democracy and together with Lord Alton visited refugee camps and homes in Albania in September 1999.

In March 2015 it was heard at Southwark Crown Court that McMahon was one of two bishops responsible for allowing Anthony McSweeney to be appointed as a priest in the Roman Catholic Diocese of East Anglia following an incident in 1998 in which "a housekeeper found what she said were pornographic images at [McSweeney's] home.

"[3] The matter was heard by McMahon, and explained to Bishop Peter Smith, and was decided upon as an incident for clergy discipline and not investigated by the police.

His personal hobbies and interests include music, reading, art, architecture, tennis and walking.