Hewlett was born in Bristol, and studied at the West of England College of Art and in Paris at the Ecole des Beaux Arts before entering the Slade School of Art in London. From 1957 he lived in Cornwall, and from 1958 to 1981 was Head of Painting at the Falmouth School of Art, working closely with Michael FINN, the Principal, and then Tom CROSS during part of his tenure.

In 1977 he was Gregynog Arts Fellow at Newtown's campus, University of Wales, and then after retiring from teaching he returned to full-time painting and ceramic sculpture and returning to Falmouth (See Hardie 1995 100 Years in Newlyn: Diary of a Gallery for a colour plate illustrating one of his large-scale ceramics. Many provincial galleries hold his work, as does the Ulster Museum, Belfast and the National Library of Wales.

His obituary by Marcus Hewlett in the Independent (30 April 2012), relates the genuine and vibrant character of the artist: 'Hewlett had a deep affinity and connection with the intense piercing beauty of the Cornish countryside, which permeates much of his work. A committed environmentalist and nature lover, in the early 1970s Hewlett founded and for several years led the Falmouth Civic Society, which continues to flourish, and was active in both heritage and environmental conservation. A lifelong pacifist, Hewlett was an active peace campaigner and took part in some of the seminal protests of the 1960s and 70s, including the Aldermaston marches.'

He was married to Liz, and had three children.

Since 1981 the artist has returned to earlier themes of interest, including the Bristol Empire and Notre Dame Cathedral, Paris. A major retrospective of his work was held at the Falmouth Art Gallery in Autumn 2000.