William Stewart Johnstone, born on 10 Mar 1869, was the eldest son of William John Johnstone (1844-1891) and his wife Mary Elizabeth Groom (1841-1911). His grandfather, William Johnstone, was the founder of Johnstone & Wilmot Pty Ltd. In 1874 when his grandfather died William John Johnstone took over the company and after his death in 1891 his brother-in-law Stuart Eardley-Wilmot was the owner for nearly 10 years. It was then formed into a proprietary company with William Stewart Johnstone as one of the managing directors with Eardley-Wilmot and William Percy Dobson.
William was educated at the Launceston Church Grammar School, before entering the family firm at the age of 17. He married Minnie Susan Frean on 2 Nov 1898 at St John's Church, Launceston. They had one daughter, Joan, born on 18 Nov 1901. William died aged 71 on 27 Sep 1940 after serving over fifty years with the company. Minnie passed away aged 81 on 12 Aug 1956.