Henry Gellibrand entered Trinity College, Oxford in 1615 where he was introduced to mathematics by Savile. He received a B.A. from Trinity College in 1619 and an M.A. in 1623. He became a friend of Briggs while in Oxford.
He entered the church becoming a curate in Chiddingstone, Kent a couple of years after receiving his M.A. from Oxford.
Gellibrand succeeded Gunter to the chair of astronomy in Gresham College, London in 1627. It was largely through the influence of Briggs that he received this chair.
Gellibrand's most famous scientific discovery was the change over the years in magnetic declination. He also made mathematical contributions to navigation, in particular working on methods to determine longitude. His methods were based on observing various celestial events and were published in Appendix concerning Longitude (1633).
Gellibrand also published logarithm and trigonometrical tables. After his friend Briggs died in 1630, he worked to complete Briggs' Trigonometria Britannica which he did, publishing the work in 1633.
Several of Gellibrand's publications appeared after he died. Institution Trigonometrical (1638) with an expanded version in 1658, applied trigonometry to navigation and astronomy. Epitome of Navigation first appeared 62 years after his death; a rather remarkable length of time.