Born Ronald Moodnick in London, he has worked in a variety of genres, but is perhaps best known for his starring role as the villainous Fagin in the musical Oliver!, based on Oliver Twist by Charles Dickens. He created the role in the original West End production, and reprised it on Broadway and in the 1968 film version, for which he was nominated for the Academy Award for Best Actor. He has appeared in several children's television series, including The Animals of Farthing Wood, Telebugs, and the Discworld series.

Moody played French entertainer and mime artist The Great Orlando in the 1963 Cliff Richard film Summer Holiday.

In 1969, he was offered, but declined, the lead role in Doctor Who, following the departure of Patrick Troughton from the part. He played Edwin Caldecott, an old nemesis of Jim Branning in EastEnders. In 2005, he acted in the Big Finish Productions Doctor Who audio play Other Lives, playing the Duke of Wellington.

A bachelor for sixty-one years, Moody married yoga instructor Therese Blackbourn in 1985. They are the parents of six children, the youngest born when Moody was seventy-three.

He is also the cousin of the Coronation Street director, Laurence Moody.