Thomas's was founded in 1971 when Joanna Thomas, an actress and mother of three young children, started the Ranelagh Kindergarten in a Pimlico church hall (now Thomas's Kindergarten, Pimlico). Six years later, the success of the Kindergarten and the strong demand for London day schools of quality encouraged David Thomas, a former Gurkha Regiment Officer to join his wife in founding a school for older children. Following the acquisition of a building in Cadogan Gardens in 1977, the "Kensington Court Lower School" came into being with two teachers and eleven pupils. In less than four years the school, now named "Thomas's", had over two hundred children. Demand for places continued to increase and, in response to this, Thomas's expanded in 1981 into two buildings in Cottesmore Gardens, just south of Kensington Gardens, to create Thomas's, Kensington.
By the end of 1989 the Preparatory School at Cadogan Gardens needed more space and in August 1990 the freehold was acquired of the former Sir Walter St John's Grammar School in Battersea. The school, which became Thomas's, Battersea, moved to its extensive new premises in October 1990 and the leasehold buildings in Cadogan Gardens were sold.In September 1992 Thomas's Kindergarten, Battersea was started in the crypt of St Mary's Church. At the end of 1992, with an increasing demand for places, the freehold was secured of the magnificent former Clapham County Girls' School in Broomwood Road and the new Thomas's, Clapham opened in September 1993.
In September 1995, David and Joanna Thomas's two sons, Tobyn and Ben, were appointed Principals, having worked in the schools for a number of years. In June 1998, a charitable foundation was formed. The aims of the charity, Thomas's Schools Foundation, are to assist certain parents financially with the education of their children and to run local community projects. In 1999, David and Joanna Thomas retired from active management of the schools, which are now run by Tobyn and Ben. Ben is also Headmaster of Thomas's, Battersea. In 2000 and 2001, a £4m construction programme was carried out to enhance the facilities at Thomas's, Clapham and Thomas's, Battersea. In 2002, an opportunity arose to improve Thomas's, Kensington through the acquisition of a nearby school, Lady Eden's. Lady Eden's was closed in Summer 2002 and its buildings converted to become the Lower School of Thomas's, Kensington. The existing buildings in Cottesmore Gardens were extensively refurbished and now house the Preparatory School. The premises of Thomas's, Kensington are freehold.
In 2004, Thomas’s acquired a long lease on the buildings formerly owned by the London Institute for the Chelsea College of Art and Design in Hugon Road, SW6. The school, which overlooks South Park, was built from 1892 and was opened on the 29 October 1894 in order to educate 1,200 children. The buildings have 35,000 square feet (3,300 m2) and are set on a 1-acre (4,000 m2) site. A £1.3m refurbishment programme by the landlords commenced in January 2005 and Thomas's, Fulham opened in September 2005 as a co-educational preparatory school for children from 4 to 11 years old. The school is expected to grow from Reception upwards up to 400 pupils by 2012.
Pupils are required to wear a uniform, which changes for the summer and winter. It consists of Navy Blue trousers or skirt for girls, a blue shirt for boys and in the winter a blue jumper for boys, and in the summer a light blue dress for girls. Only boys wear insignia, with a unicorn on the jumper and a snake clasp belt.