The College was founded in 1532 (Old Style) in the will of Richard Collyer, who was born in Horsham, and became a wealthy member of The Mercers' Company of the City of London.

Collyer willed that one of his houses in the City, variously called 'The Sonne' or 'The Sunne', be sold and the proceeds used to build a school-house in Horsham for "the nomber of thre score scholars".

However, it was extended then rebuilt in 1660, in order to accommodate "neare an hundred scholars ... with diligence and good success" by 1666, such that none of Collyer's original structure survives.

)[3] In the eighteenth century it fell into disrepair, such that the Mercers' Company surveyor reported that it would cost £1,040 to renovate, "but you will still have a very old and imperfect building."

Accordingly, in 1840 the second building was demolished (save for the part incorporated into Arun House), and a new, late-Elizabethan style structure built for the sum of £2,240.

[2] By the late nineteenth century, the population of Horsham had expanded to 10,000 (accelerated by the coming of the railway and its associated employment), the City and Guilds Institute (which the Mercers' Company had helped found) decreed that education needed to be extended to include the new sciences, and money needed to be found to replace the school buildings yet again.

[2] The present site in Hurst Road was found, and the current building was designed in 1892 by Arthur Vernon,[4] and built by Joseph Potter in 1893[5] for a contract price of £5,795.

In the 1890s Collyer's taught 110 boys from ages 7 to 17, both boarders and day-schoolers, and for the first time included a sixth form to study for university entrance.

[2] Collyer's ceased to accept boarders in 1935, and the dormitories were converted into much-needed library and common-room space.

(The final phase of building work, to be a new classroom block replacing the World War Two emergency wooden huts, was never undertaken.)

In 2006 work began to extend the Sports Hall, or Cowley Building, to provide more teaching and social space.

Since then the college's principals have been: After recovering from a country-wide low point in schooling in the eighteenth century, headmasters from William Pirie to George Thompson successively raised standards, such that in 1904, when the school had 110 pupils, six OCs held open awards at Oxford or Cambridge, and another was a City and Guilds scholar.