Whanganui Collegiate School was founded as Wanganui Collegiate School by a land grant in 1852 by the Governor of New Zealand, Sir George Grey, to the Bishop of New Zealand, George Augustus Selwyn, for the purpose of establishing a school. It was originally a boys-only school but in 1991 began admitting girls at senior levels and went fully co-educational in 1999. The school celebrated its 150th anniversary in 2004. St George’s School moved to the Collegiate campus in 2010. The combined schools provide primary education for day students on the St George campus, and secondary education for day and boarding students on the Collegiate campus. Collegiate is an International Member of The Headmasters' and Headmistresses' Conference which represents heads of the leading independent schools in Ireland, the United Kingdom and international schools mainly from the Commonwealth. Whanganui Collegiate is one of only three member schools in New Zealand. Collegiate is one of three Round Square Schools in New Zealand along with King's College and Christ's College, Collegiate was made a member in June 2019. Prince Edward, Earl of Wessex, spent two terms in 1982 at the school as a junior master during his gap year. In November 2012, it was announced that the school would integrate into the state system effective January 2013, after requiring a $3.8 million bailout from the government to stay afloat. In January 2019, the Whanganui Collegiate School Board of Trustees and Whanganui College Board of Trustees unanimously voted to add an 'h' to the spelling of 'Wanganui', following other local organisations and secondary schools after the Whanganui District was similarly renamed in November 2015.
As a boarding school, the house system plays a significant role in student life. Each house accommodates approximately 80 students, and each has its own Housemaster, Assistant Housemaster and Matron. The school houses are named Harvey, Hadfield, Grey, Selwyn, Godwin and Bishops. The school grounds also host numerous sporting facilities, including the Izard Gymnasium, High Performance Cricket Centre, A Water Surface Hockey turf a full-sized Cross Country Course and many team sports fields. The nearby Whanganui River is used by students for rowing training and competitions; Rowing being one of the sports in which Collegiate has traditionally excelled, having won the Maadi Cup 17 times, a national record. The Collegiate women's squad had a particularly strong year in 2006 when it won the most prestigious women's race in the lower north Island, the Levin Jubilee Trophy, for the first time. Unfortunately, such success could not be replicated on the national stage, with the u18 girls eight only managing bronze in the penultimate race of the 2007 Maadi Cup. The School also hosts the nationally popular Whanganui Cricket Festival each year which sees over 1000 cricketers display their skills throughout the month of January. Since 1925, the school's 'First XV' rugby team has played Christ's College, Wellington College and Nelson College in an annual quadrangular rugby tournament, this Tournament is played at a different school every year playing at Collegiate once every four years. In recent times, this tournament has been dominated by Wellington College. Whanganui Collegiate last won in 1991.