Marshall was educated at Humberstone Foundation School (also known as Clee Grammar School for Boys and became the comprehensive Matthew Humberstone School in September 1973) on Clee Road in Old Clee, Cleethorpes, and Magdalen College, Oxford, where he took double first class honours in mathematics and was awarded a University Junior Mathematical Prize in 1961.
He was then organist of Old Clee parish church, and was supplementing his modest income by playing in the local theatre and by working as a compass adjuster.