Gallaher was born in Brisbane and educated at the Anglican Church Grammar School.[2] and studied at the Queensnsland Conservatorium of Music[3]. In 1977 he appeared on the Mike Walsh Show[4] , he had his own television program, The Simon Gallaher Show on ABC-TV (1982-83), in which he sang and played the piano.[3] Later, he joined the Queensland Light Opera Company.[5]

With his own musical theatre company Essgee Entertainment, Gallaher has produced and acted in stage productions of The Pirates of Penzance, H.M.S. Pinafore, The Mikado, The Merry Widow and A Funny Thing Happened on the Way to the Forum, among others.