Lewis published her first novel in 2003 and it won the 2004 Tir na n-Og Award. She became better known when her novel Martha, Jac a Sianco won the Wales Book of the Year award in 2005. Her tenth novel Y Bwthyn won in 2016 and Y Gwreiddyn was shortlisted for Welsh Book of the Year Award 2017. Martha, Jac a Sianco was adapted for film which won the Atlantis Prize at the 2009 Moondance Festival in Colorado. Lewis has also written for television, including Hinterland and Hidden. Lewis now lives on the outskirts of Aberystwyth.
