John Stevens Henslow (6 February 1796 – 16 May 1861) was an English Anglican priest, botanist and geologist. He is best remembered as friend and mentor to his pupil Charles Darwin.
Henslow was born at Rochester, Kent, the son of a solicitor John Prentis Henslow, who was the son of John Henslow.
Henslow was educated at St. John's College, Cambridge where he graduated as 16th wrangler in 1818, the year in which Adam Sedgwick became Woodwardian Professor of Geology.
Henslow graduated in 1818. He already had a passion for natural history from his childhood, which largely influenced his career, and he accompanied Sedgwick in 1819 on a tour in the Isle of Wight where he learned his first lessons in geology. He also studied chemistry under Professor James Cumming and mineralogy under Edward Daniel Clarke. In the autumn of 1819 he made valuable observations on the geology of the Isle of Man (Trans. Geol. Soc., 1821) and in 1820 and 1821 he investigated the geology of parts of Anglesey, the results being printed in the first volume of the Transactions of the Cambridge Philosophical Society (1822). The Philosophical Society was founded in November 1819 by a group at Cambridge with Professors Farish, Lee, and Sedgwick and Henslow (at that time not yet a professor). The idea and initial impetus for the society originated from Sedgwick and Henslow.
Meanwhile, Henslow had studied mineralogy with considerable zeal, so that on the death of Clarke he was in 1822 appointed professor of mineralogy in the University of Cambridge. Two years later he took holy orders. Botany, however, had claimed much of his attention, and to this science he became more and more attached, so that he gladly resigned the chair of mineralogy in 1827, two years after becoming professor of botany. As a teacher both in the classroom and in the field he was eminently successful. He was a correspondent of John James Audubon who in 1829 named Henslow's sparrow (Ammodramus henslowii) after him.