Gallaher was born in Brisbane and educated at the Anglican Church Grammar School. and studied at the Queensnsland Conservatorium of Music. In 1977 he appeared on the Mike Walsh Show, he had his own television program, The Simon Gallaher Show on ABC-TV (1982–83), in which he sang and played the piano. Later, he joined the Queensland Light Opera Company.
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.
For most of the 1980s his song "Australia Be Proud" was heard at the conclusion of each broadcast day on television station CTC Canberra, accompanied by a montage of video clips from across Australia.
In 2014 Gallaher returned to the stage to be reunited with Jon English for the first time in many years as they performed in Spamalot for Harvest Rain Theatre Company at the Queensland Performing Arts Centre (QPAC). In 2015 he took over the role of The Wonderful Wizard of Oz in the Australian production of Wicked and opened with it at the commencement of the Brisbane season. Gallaher recently directed a new production of his version of The Pirates of Penzance, which played at the QPAC to celebrate the 30th Anniversary of the Arts Centre and of the show's original opening.
He is currently Deputy Chair on the Board of Trustees of QPAC and was on the board of the Queensland Theatre Company (QTC). , He was formerly the Chair of Harvest Rain Theatre Company (2012–15). He is patron to many arts organisations including The Queensland Show Choir, Queensland Youth Symphony Orchestra, Spotlight Theatre Company, Gold Coast Theatre Alliance and the Canberra Area Theatre Awards. He is a member of Live Performance Australia (LPA) and sits on the Helpmann Awards nominating panel for Musical Theatre.