The music manuscripts, 1927-1986, of Mansel Thomas, comprising music for male voice choirs, mixed choirs, youth and female voices, solo songs, and instrumental compositions. They consist mainly of Christmas music, hymn-tunes, liturgical music, arrangements of Welsh traditional melodies, secular music and solo songs, the majority of which have been published by the Mansel Thomas Trust and the Cwmni Cyhoeddi Gwynn Publishing Company. Some manuscripts are in the hand of copyists and may contain later notes and additions.

A manuscript music book which belonged in 1898 to Richard Jones, Ty Uchaf, Llanfrothen, and which contains carols, etc.