In the 1960s, he was commonly referred to as the most often commissioned composer in Britain, and over his lifetime he produced more than 250 works in a wide variety of genres.

[4] He wrote a number of pieces connected to his royal post, including Mass of Christ the King (1978) (see below) and Lament in Memory of Lord Mountbatten of Burma (1980).

Some of Williamson's early works use the twelve-tone technique of Arnold Schoenberg, but his greatest influence is often said to be Olivier Messiaen.

His operas include English Eccentrics, to a libretto by Edith Sitwell; Our Man in Havana, after the novel by Graham Greene; The Violins of Saint-Jacques, from Patrick Leigh Fermor's novel; and two adaptations of plays by August Strindberg, Lucky Peter's Journey after Lycko-Pers resa [sv], and The Growing Castle after A Dream Play.

The cassation The Valley and the Hill was written for the Silver Jubilee of Queen Elizabeth II in 1977 and performed by 18,000 children.

The composer's largest choral work, his Mass of Christ the King, was commissioned by the Three Choirs Festival, also for the 1977 jubilee.

For example, in 1988 Williamson wrote a large-scale choral-orchestral work The True Endeavour, the orchestral Bicentennial Anthem, the Fanfare of Homage for military band, a ballet Have Steps Will Travel for John Alleyne and the National Ballet of Canada, Ceremony for Oodgeroo (Oodgeroo Noonuccal, formerly known as Kath Walker) for brass quintet, and also commenced work on a substantial new choral-symphony The Dawn is at Hand (to texts by Kath Walker), completed and performed in Australia the following year.

The same year also saw the premiere of an orchestral work With Proud Thanksgiving, commissioned for the fiftieth anniversary of the United Nations, and dedicated to the memory of Williamson's long-time friend, the UK Prime Minister Harold Wilson.

At the funeral of Queen Elizabeth II, held on Monday, 19 September 2022, mourners filed into Westminster Abbey to 'O Paradise' by Williamson.

It is not clear why Williamson did not qualify for a substantive award, as there appears to be nothing on the public record to suggest he ever relinquished his Australian citizenship.