Harrington was born in London, England, on March 13, 1928, the oldest sibling of a household that produced seven children. He received his public education at The Latymer School graduating in 1946 and later enrolled in the Royal Academy of Dramatic Arts where he excelled in studying both acting and music. He graduated in 1950. His first professional beginnings in the entertainment industry began singing in the background chorus of the famed Palace Theatre in London. One fateful night in 1952, he happened to be discovered by famed motion picture director David Lean and his then wife, actress Ann Todd, who were so impressed by young Aidan's vocal abilities and charming looks, that they plucked him from obscurity and arranged for him a motion picture contract through London Film Productions where he would make his debut, under Lean's direction and with Todd as his first leading lady, in 'The Sound Barrier' (1952). Young Aidan's first screen appearance was so successful that within the very first year of his contract he appeared in 10 more films, one of which included the lavish costume epic 'Ivanhoe' whose cast included such acclaimed talents as "Robert Taylor", "Elizabeth Taylor", and "Joan Fontaine".