In 1875 Brettell became player-secretary-manager of Liverpool side St. Domingo, later being one of the founder members Everton, initially as a player (playing primarily at full-back) but later becoming assistant secretary and then secretary. He also worked as a reporter for the Liverpool Mercury.
He joined Bolton Wanderers as secretary in 1896 before becoming the first (secretary-)manager of Tottenham Hotspur in February 1897. He signed a number of players from Bolton, but left to become Portsmouth's first manager in May 1898. He again signed a number of players from the North of England and led Portsmouth to second place in their first ever season in the Southern League. He left Portsmouth in June 1901.
In 1903 Brettell became the first manager of Plymouth Argyle, leaving in 1905.