He is chiefly remembered for his detailed phenology observations of the times of year at which events in natural history occurred.

He was the youngest son of George Leonard Jenyns of Bottisham Hall, Cambridgeshire, a magistrate, landowner and a prebendary of Ely Cathedral.

His father had inherited the Bottisham Hall property on the death of his distant cousin Soame Jenyns (1704–1787).

[2] Jenyns was the original choice for the naturalist on the second voyage of HMS Beagle but turned down the offer due to ill health and parish duties.

Fitzroy, as Naturalist, in the Beagle, on his voyage to survey the coasts of S.America, afterwards going round the globe:- declined the appointment wc was afterwards given to Charles Darwin Esq.

[2] In 1825 Jenyns gave his first paper on the "Ornithology of Cambridgeshire" to the Cambridge Philosophical Society, where he became a member and then a fellow.

[7] The museum archives contain extensive material from Jenyns, including manuscripts and books on local natural history.