Mansel Treharne Thomas OBE (1909 – 1986) was a Welsh composer and conductor.
He was born at Tylorstown, Rhondda, in a house on which a plaque was later erected by the Rhondda Civic Society. At sixteen, he won a scholarship to the Royal Academy of Music, where he studied under Benjamin Dale. As a composer, he specialised in songs and musical settings of poetry. He joined the BBC in 1936, but interrupted his career to serve in World War II. From 1946 until 1965, he was the principal conductor of the BBC National Orchestra of Wales and the BBC Welsh Chorus. After suffering a stroke in 1979, his career effectively ended. He died in Abergavenny, Monmouthshire, seven years later at the age of 77.
The Mansel Thomas Trust was established in 1987 to commemorate the composer. It is concerned mainly with collecting his works and making them available to musicians.