Lewis was born in Neath, where he joined the church choir and began learning the organ. He studied at the University of Wales, Cardiff, under Alun Hoddinott; with György Ligeti and Karlheinz Stockhausen at Darmstadt; with Bogusław Schaeffer in Krakow and with Don Banks in London. He taught at Leeds College of Music and the University of Wales, Bangor, under William Mathias. Early performances included Fanfares with Variations and the Chamber Concerto with the BBC Welsh Orchestra under John Carewe, and, at the 1967Cheltenham Festival, his Two Cadenzas for piano and Epitaphium - Children of the Sun for narrator, chamber choir, piano, flute, clarinet and percussion. BBC commissions included the orchestral works Mutations I, Aurora, Scenario, Praeludium, Memoria and Limina Lucis. Other works include a Piano Concerto, Duologue for violin and piano, Scena for violin and piano , Teneritas for flute and piano, Sonante for clarinet and piano, Cantus for clarinet and piano, a Piano Trio, a Wind Quintet, several mixed ensemble pieces including Time-Passage, Stratos, Epitaph for Abelard and Heloise and Litania, piano works including Tableau, Fantasy, Threnody, Trilogy, Musica Aeterna and Sereno, organ works including Mutations II, Momentum and Esultante, Dreams, Dances and Lullabies for harp, Silentia Noctis for high voice and piano, and many choral works, notably Carmen Paschale, Hymnus Ante Somnum, Sequentia ad Sancte Michaele, Westminster Mass, Recordatio and a sequence of eleven Sacred Chants. Lewis's music is characterised by rhythmic energy and harmonic complexity alternating with extreme stillness; latterly his music has tended towards greater simplicity and tranquility and it often appears to inhabit a twilight world of dreams. However, he has achieved this simplicity through a process of constant refinement of his compositional technique, rather than any rejection of his earlier complexity. In 2005 Ivan March in Gramophone magazine described the CD release of Epitaph for Abelard and Heloise, Litania and Musica Aeterna as "...a disc not to be missed by anyone who cares about communicative 20th-century music."
