Master William Andrew Johnston was born in Croydon, Surrey, England on 14 September 1903.
He was the son of Scottish parents, Andrew Emslie Johnston (b. 1876), a plumber, and Elizabeth "Eliza" Watson (b. 1876) who hailed from Aberdeenshire and the Isle of Skye, respectively, and who had married in Glasgow in 1902. He had one sibling, his younger sister Catherine Nellie (b. 1905).
William appears with his family on the 1911 census when they were living at 21 Luna Road, Thornton Heath, Surrey. Also living in England, but in the county of Sussex, was his maternal aunt Margaret Ann Ford. Margaret's daughter Frances, was already in the United States working as a domestic servant, and so impressed the family with tales of a better life that both families decided to leave for America. Travelling with them was a friend of the Watsons, Alice Harknett. The Johnstons originally intended sailing in October 1911 but decided to wait for the Fords. Their passage was originally booked on the Philadelphia, but owing to the coal strike a further postponement had to be made, and the party ultimately sailed on the Titanic which boarded at Southampton as third class passengers (ticket number W./C. 6607 which cost £23, 9s).
The entire party of ten were lost in the sinking. Their bodies, if recovered, were never identified.