Thomas was born in 1977 in Gorseinon near Swansea, the daughter of M. Wynn Thomas OBE, a Professor of Literature at Swansea University, and Karen Thomas. She was educated at the Welsh-speaking Ysgol Gyfun Gŵyr in Gowerton near Swansea, and by the time she was fifteen was singing in the Swansea Bach Choir. She won a choral scholarship to Clare College, Cambridge University, where she gained a starred first in Anglo-Saxon, Norse and Celtic, and completed an MPhil. After auditioning to Sir John Eliot Gardiner she joined the Monteverdi Choir and in the year 2000 sang much of the Bach Cantata Pilgrimage. In 2001 she moved to pursue postgraduate vocal studies at the Royal College of Music in London.
She went on to sing with The Sixteen, Polyphony, Cambridge Singers and The Gabrieli Consort, as well as pursuing a solo career. She is the first singer ever to record Bach’s Alles mit Gott, a birthday ode written in 1713 and discovered in 2005. She first received great acclaim for her ‘Pie Jesu’ on Naxos’ award-winning recording of the Rutter Requiem.
In 2006 Elin married baritone opera-singer Robert Davies. The couple live in Brighton in Sussex, and have two sons.