Gentleman studied at the St Albans School of Art and the Royal College of Art, under Edward Bawden and John Nash. Gentleman has worked in various media - watercolour, lithography, wood engraving - and at scales ranging from postage stamps to platform-length murals for Charing Cross tube station (London). He has written and illustrated six books about cities and countries, several for children, and two about his own work. Throughout his working life he has also travelled widely, drawing and painting throughout Britain, Europe and India.

His early drawings and wood engravings included paperback covers, book illustrations and designs for textiles and wallpapers. In the sixties he illustrated American limited edition books and four children's’ books, and designed the first of 100 postage stamps; in the seventies, he made many lithographs, and designed posters for the National Trust and the Charing Cross tube station mural. In the eighties and nineties he wrote and illustrated in watercolour David Gentleman’s Britain and five companion books on London, the British coastline, Paris, India, and Italy. From 2003-7 he designed a number of anti-Iraq war protest posters.

There have been many exhibitions of his watercolours and lithographs. His work is in public collections including Tate Britain, the British Museum, the Victoria and Albert Museum and the Fitzwilliam Museum. He is a Royal Designer for Industry (RDI) and a member of Alliance Graphique Internationale (AGI). In 2007 he was awarded the Prince Philip Designers Prize.