He was apprenticed at the age of fifteen to Joseph Armstrong at the Great Western Railway's Wolverhampton Stafford Road Works.

A year later, Joseph Armstrong was promoted to the position of the GWR's Chief Locomotive Engineer and moved to Swindon Works.

George Armstrong, Joseph's brother, succeeded him as Northern Division locomotive superintendent, with Dean under him as Stafford Road works manager.

Dean was the son of a soap works manager in New Cross, London, and attended the Haberdashers' Boys' School.

[1] Dean was ill during his final years as Chief Locomotive Engineer, and he increasingly allowed Churchward to take on the day-to-day responsibilities.