[1] Henslow was educated at St. John's College, Cambridge where he graduated as 16th wrangler in 1818,[2] the year in which Adam Sedgwick became Woodwardian Professor of Geology.

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.

[1] From 1821 Henslow had begun organising a herbarium of British flora, supplementing his own collecting with a network which expanded over time to include his friends and family, and the botanists William Jackson Hooker and John Hutton Balfour, as well as about 30 of his students.

Henslow is remembered as friend and mentor to his pupil Charles Darwin, and for inspiring him with a passion for natural history.

Seeing a perfect opportunity for his protégé, Henslow wrote to the ship's captain Robert Fitzroy telling him that Darwin was the ideal man to join the expedition team.

[4] Besides Darwin, other famous students of Henslow included Miles Joseph Berkeley, Cardale Babington,[5] Leonard Jenyns,[6] Richard Thomas Lowe and William Hallowes Miller.

[9] Henslow did not resign his chair, and continued to give lectures, set and mark exams, and take part in university affairs.

In 1843 he discovered nodules of coprolitic origin in the Red Crag at Felixstowe in Suffolk, and two years later he called attention to those also in the Cambridge Greensand and remarked that they might be of use in agriculture.

[1] In 1851, Carl Ludwig Blume named a genus of flowering plants belonging to the family Santalaceae, from Indo-China as Henslowia, in honour of Henslow.

[15] Their sons included George Henslow (1835–1925),[5] who became the Royal Horticultural Society's Professor of Botany[16] and the first President of the Churchmen's Union for the Advancement of Liberal Religious Thought.