Keyl was born in Frankfurt am Main, son of a wine merchant, and showed at an early age an interest in drawing animals. He studied at the Städelsches Kunstinstitut in Frankfurt under Jakob Becker, and later was a pupil of Eugène Verboeckhoven in Brussels.

From 1847 he lived permanently in England, and was naturalized in 1858. He was the only pupil of Edwin Landseer, and through him Keyl was introduced to Queen Victoria and Albert, Prince Consort. He obtained many commissions from the royal family, producing oil paintings, watercolors, sketches, and painting photographs; a notable commission was to paint Looty, the Queen's Pekingese. Apart from royal patronage, work included book illustrations, and plates for the Illustrated London News.