The School's first headmaster in 1843, William Randall Lee, established the school in Brighton.  It was known at first as Connaught House but almost half a century later two of the founder's sons brought the school here to Ashdown House where its centenary on this site was celebrated in 1986.

The estate here is an ancient one indeed: records show that a chapel was consecrated here in 1296.  The Tudor manor which still forms one wing of the school dates from about 1575 but it was not until 1794 that Latrobe finished our famous central block before he won even greater glory in America.

In 1910 the school was taken over by Arthur Evill, whose wife Kathleen was a grand-daughter of W.R. Lee, and they were succeeded in 1934 by Aidan and Joan Wallis, who steered the school through the difficult years of the war and the unwelcome publicity of two bombs narrowly missing the house.  Field Marshall Lord Slim, whose children were here, supported the decision to stay put and the school flourished.
When Aidan Wallis died in 1949 the school was taken over by the great W.G. Williamson, whose teaching of the Classics soon became legendary.  In his twenty five year 'reign' the school's numbers doubled and its scholarship record became one of the finest in the country.

In 1975 Ashdown House became a charitable Trust with a board of governors.  At this time Clive Williams, himself an Old Boy of the school and one of W.G.W.'s Eton scholars, took over with his wife Rowena, becoming the sixth headmaster.  The school also became co-educational for the first time in its history.

In 2003, Robert Taylor took over the running of the school as the eighth headmaster.

In 2009 Dominic Floyd and his wife Maria joined the school.  Dominic is the ninth headmaster in the one hundred and seventy years of the school's existence.  Ashdown also merged with the Cothill Educational Trust.