Between 1799 and 1803, he acted as Bewick's principal assistant on the second volume of his A History of British Birds.

After completing his seven-year apprenticeship with Bewick he moved to London in 1804,[1] where he married a daughter of the copper-engraver Charles Turner Warren (1762–1823).

In 1814, he received from the Earl of Bridgewater a commission for a large commemorative picture, Banquet for the Allied Sovereigns, at the Guildhall, London.

[2] Thirteen artists submitted works, and Clennell received one of the premiums for his picture entitled The decisive charge of the Life Guards at Waterloo.

[1] Clennell resumed work on the picture of the Allied Sovereigns in 1817, but suffered another bout of depressive mental illness, and his family found him throwing his palette and brushes at the canvas, "to get the proper expression."