Alex Davidson Johnston was the son of Scottish parents, Alexander Davidson Johnston and Margaret Smith Johnston. His father was a baker and grocer in King Street, running "The Edinburgh Bakery".

They lived in King Street, and Alex junior joined Westoe Secondary School in 1895.

He was at the school until he was about 15 when, in 1898, he was apprenticed to the Northern Press and Engineering Company. During his time he took classes at the Durham College of Science and Sunderland Technical College.

For the last two years of his six-year apprenticeship he worked as a draughtsman. He continued as a draughtsman with Northern Press, and then with Vickers, Son and Maxim in Barrow-in-Furness, and in 1906 was granted a Whitworth Exhibition.

At this time he joined the Institute of Mechanical Engineers as a graduate member.

In 1908 he moved to Manchester and joined Henry Simons Ltd, a company making grain milling equipment. Initially he was in charge of the experimental department, and in 1912 he was promoted to "Engineer-in-Charge of Milling Machine Design", at which time he became an associate member of the Institute.

Alex stayed with Henry Simons until, on 16-Dec-1915, he enlisted as a Private with the London Scottish Regiment, and was posted to France in June, 1916.

He was killed at the Battle of the Somme on 1st July 1916, and he is buried in the Gommecourt British Cemetery No 2, half way between Arras and Amiens.