Born in Dublin, where he was educated, he migrated to Australia in 1889, becoming a clerk and railway worker.

[2] In 1908 he was elected to the Legislative Assembly of Queensland as the Labor member for Charters Towers, where he remained until 1912.

He remained in the Senate until his defeat in 1917,[2][3] after which he returned to the Queensland Legislative Assembly as the member for Flinders in 1918.

In 1932, he changed seats, moving to Carpentaria,[1] and resumed his position as Attorney-General, which he retained until 1940.

Mullan died in 1941 and was accorded a State funeral which took place from St Stephen's Cathedral to the Toowong Cemetery.