Born to a Sephardic Jewish family from Morocco and Menorca,[2] he trained as a lawyer at Middle Temple and was called to the Bar of England and Wales in 1939.

In September 1942, a group of fellow Gibraltarians, clerks and workers, led by trade-unionist Albert Risso, came together to form an association advocating the return of the evacuees and the civil rights of the local inhabitants of Gibraltar.

In an election to the Gibraltar Legislative Council held on 19 September 1956, four of the seven winning candidates were from the AACR, Hassan, Abraham Serfaty, Albert Risso, and J. E.

During the 1960s, Hassan addressed the United Nations Special Committee on Decolonization twice, opposing Spain's claim for Gibraltar and insisting that the vast majority of Gibraltarians wanted to remain British.

He resigned without completing his term as Chief Minister in 1987 after an agreement on the shared use of Gibraltar Airport was signed by Spain and the United Kingdom, citing personal reasons.

[2] Hassan's Triumphant Return to Gibraltar in 1963 when he and Peter Isola were cheered by crowds in John Mackintosh Square has been recorded in a commissioned painting by Ambrose Avellano.