[1] Born on 18 January 1868 near Magherafelt in County Londonderry, he was the eldest son of the seven children of James Johnston and his wife, Sarah (née Davidson).
[1][3] He stood as a Liberal Home Rule candidate for South Londonderry at the December 1910 general election but was defeated by the Liberal Unionist MP John Gordon.
[1] He became a member of the Pembroke Urban Council in 1911.
[2] 1894 he married Kathleen King; they had one child, Denis Johnston, born in 1901.
[1] William John Johnston died in Dublin on 29 November 1940, aged 72.