His specialisation in domestic architecture and regular commissions saw him develop a romantic style that found more favour with his clientele and often included a number of recurring motifs such as red and white striped deckchairs, and items of mechanical engineering such as hot air balloons, paddle steamers, railways, trains, trams, and lighting fixtures.

Herbert Read, the writer and art critic, bought a painting and invited Kelly to illustrate the second edition of his short novel, The Green Child.

Read returned the favour by writing the introduction to the book Paintings by Felix Kelly published in 1946 by Falcon Press.

His personality, wit, and charm ensured he was often invited back by his wealthy clients and, in some cases, developed into lifelong friendships.

These murals were commissioned by George Howard and paid for with the location fee from the Brideshead Revisited television production.

He produced an artist's impression of a Palladian temple for Sebastian de Ferranti, who then worked with various architects to realise the design.

de Ferranti chose Julian Bicknell to build the house which was completed in 1986[9] Later Kelly worked on the Cave, a modern grotto, at Henbury Hall.

His tendency towards the surreal and sinister was reflected in the dust jacket work he did for Faber and Faber's Best of series including Best Horror Stories (1957), Best Detective Stories (1959), and Best Tales of Terror (1962), as well as a beautiful wrap-around dust jacket - plus all internal illustrations - for Haunted Houses (1956) by Joseph Braddock.