St. Johns Mansion has had a history spanning almost 900 years; in the mid-12th century, during the reign of Henry II, the land on which St. John's House stands was given to the establishment of the Hospital of St. John the Baptist. William de Beaumont brought the hospital, then Earl of Warwick. This hospital provided two purposes: To help the local poor and ill and provide casual overnight boarding and food to impoverished travellers such as pilgrims. The Hospital of St. John the Baptist was one of two such hospitals in the town of Warwick at the time. The other was the Hospital of St. Michael, founded with the sole purpose of providing help and respite to those in the parish suffering from leprosy. Of both hospitals, only the chapel building of St. Michael still stands. In 1291's taxation, the hospital-owned a dovecote worth 2 shillings.

Additionally, the carucate of land owned by the Hospital was valued at 10 shilling per year. In 1337, protection was granted to the hospital's brethren and their attorneys to collect alms at churches. At this time, it was suggested that some building renovation was necessary. It is known that in 1610 the site comprised four standing buildings, including a gatehouse topped with crenulations. The largest of the three other buildings has crossed at the roof's apex, suggesting its religious use as the site's chapel. The hospital site also included a cemetery - remains have often been dug up during refurbishment or remodelling works on the House. The first recorded case was in the 1830s when work was undertaken in the kitchen garden. In 1987, two workers digging to the Coten End discovered two skulls in front of St. John's Court flats.

In 1791, the building was rented out for the first time by the Earl of Warwick for public use to convert it into a school. The school, then known as St John's Academy, was founded by William T Fowler and was set up as a school for "Young Gentlemen" (as advertised on the hand-bill[6]. Throughout the life of the school, its cohorts changed frequently. In 1828, the daughters of William Fowler, then running the school, changed it to a school for girls. It was then reverted in 1845 under a Mr. Townsend. Then it returned to a girls' school in 1884, which continued until the very end of the 19th Century. As money became tighter in the latter part of the school's life, the school restricted itself to the lower part of the house, with upper rooms being leased out to local artists and other public figures. Their studios were open for public viewing. The school was declared bankrupt in 1900 and closed down.