William Mathias CBE (1 November 1934 – 29 July 1992) was a Welsh composer.
Mathias was born in Whitland, Carmarthenshire. A child prodigy, he started playing the piano at the age of three and began composing at the age of five. At Aberystwyth University, Mathias was a member of the Elizabethan Madrigal Singers and wrote 'Gloria in Excelsis Deo' for them in 1954. He studied at the Royal Academy of Music under Lennox Berkeley, where he was elected a fellow in 1965. In 1968, he was awarded the Bax Society Prize of the Harriet Cohen International Music Award. He was professor of music and head of department in the University of Wales, Bangor, from 1970 until 1988.
His compositions include large scale works, including an opera, The Servants (1980), three symphonies and three piano concertos. Much of his music was written for the Anglican choral tradition, most famously the anthem Let the people praise Thee, O God written for the July 1981 royal wedding of the Prince and Princess of Wales, which had a television audience of an estimated 1 billion people worldwide.
Mathias wrote his Sinfonietta – initially called Dance Suite – for the Leicestershire Schools Symphony Orchestra (LSSO) in late 1966, and it received its first performance at Leicester De Montfort Hall during the 1967 Schools Festival. It was also included in the orchestra's tour programme for Denmark and Germany later in the year. The LSSO made the first commercial recording of Sinfonietta for the Pye Golden Guinea label in July 1967 under the direction of the composer (see external links below).