Croasdell grew up in Hampstead, and then studied law at Pembroke College, Cambridge.
After graduation, he ran the youth section of the League of Nations Union, and campaigned on behalf of the Republican side in the Spanish Civil War.
[1][2] During World War II, Croasdell served as a tank commander, then as an Army officer on board a ship in the Far East.
He spent time as a lawyer in private practice, but in 1950 became the legal officer of Equity.
[1] While Croasdell privately supported Marxism, as leader of the union he focused on taking a non-partisan role in negotiations.