He was born at Toxteth Park, near Liverpool in Lancashire. His father was a small farmer. He was impoverished throughout his entire brief life. He was a Calvinist and, through his connection with Emmanuel College, likely a Puritan. His means of financial support following his leaving the University of Cambridge (without a degree) was likely by holding a curacy in Hoole, Lancashire.
At Cambridge he became familiar with the works of Johannes Kepler, Tycho Brahe, and others. Horrocks was convinced that Lansberg's tables were inaccurate when Kepler predicted that a near-miss of the transit of Venus (when the planet Venus can be seen from Earth as crossing in front of the Sun) would occur in 1639. Horrocks believed that the transit would occur, having made his own observations of Venus for years.
Horrocks focused the image of the Sun through a telescope onto a piece of card, where the image could be safely observed. From his location in Much Hoole, Lancashire, he calculated that the transit was to begin at approximately 3:00pm on November 24, 1639. He first observed the tiny black shadow of Venus on the card at about 3:15pm.
His observations allowed him to make a well-informed guess as to the size of Venus, as well as to make an estimate of the distance between the Earth and the Sun. His figure of 59,000,000 miles was far from the 93,000,000 miles that it is known to be today but it was a far more accurate figure than any suggested up to that time.