In 1917, Dalby won a scholarship to Leicester College of Art and began his career as a freelance artist for various commercial firms.
[1] On 14 October 1943, during the Second World War, Dalby received an emergency commission in the Royal Air Force Volunteer Reserve, as an acting pilot officer (probationary) in the Administrative Branch.
As it was difficult to find employment as a freelance artist in post-war England, Dalby briefly worked at a blood transfusion institute in Sheffield.
Dalby illustrated The Railway Series books from the original title, The Three Railway Engines (which he re-illustrated, replacing the original art work by William Middleton), up until Percy the Small Engine.Although Dalby's illustrations didn't entirely satisfy the author, and errors in detail caused all kinds of problems, his pictures—with their bold lines, lively energy and bright, gem-like colours—quickly caught the imagination of young readers and he undoubtedly set the style for the series.
Awdry had written to Dalby, "I beg, pray and exhort you not to make Percy look like a green caterpillar with red stripes!