At the age of eighteen, Gambon entered drama school at the Royal Academy of Dramatic Art in London. He studied for three years, receiving a BA in Classical Acting. Gambon built a very solid CV whilst at RADA, including the works of William Shakespeare and Anton Chekhov. While at RADA, age nineteen, he joined the Unity Theatre in King's Cross. Five years later he wrote a letter to Micheál MacLiammóir, the Irish theatre impresario who ran Dublin's Gate Theatre. It was accompanied by a CV describing a rich and wholly imaginary theatre career – and he was taken on.
In 1998, he was knighted for his services to drama.
In 2004 he took the role of Albus Dumbledore, Hogwarts's headmaster in the third Harry Potter film, Harry Potter and the Prisoner of Azkaban, taking over from the late Richard Harris, and would go on to reprise the role throughout the rest of the film franchise.
The announcement of his casting in Doctor Who was shortly after his farewell to the Harry Potter franchise. He appeared alongside singer Katherine Jenkins, with whom he got on well. He became the second knight to appear in Doctor Who after Derek Jacobi.