William Gillilan Johnson was born at Fortfield, Belfast, in 1808. He was educated at Belfast Academy and graduated from Trinity College, Dublin.

Johnson was called to the Irish Bar in 1838 though did not actively practice law. He was Commissioner of the Peace for Counties Antrim and Down as well as Deputy Lieutenant for Down.

Johnson was elected as a Conservative MP for Belfast in 1841. He was knighted by the Queen on her visit to Belfast on 12 August 1849.

As a dedicated Anglican, Johnson donated generously to Church of Ireland causes. He laid the foundation stone of St Stephen’s Church of Ireland on 8 May 1869. After his term as mayor, Johnson devoted much of his time to the support of charitable and medical institutions including the Royal Hospital (now the Royal Victoria Hospital) and Belfast Charitable Society.

He died at his College Street residence, Belfast, on 9 April 1886.