Cranbrook opened its doors on 22 July 1918 and the first classes were held the following day. Under the auspices of the Reverend Frederick Perkins, the first Headmaster, the School Motto and crest were created, the first School Council was established and the School Song was adopted.
In the wake of World War 1, Headmaster Brigadier Iven Mackay formed the Preparatory School and formally separated the Junior School from the Senior School; in addition, three more Houses were founded and the Chapel was consecrated.
Brian Hone established St Mark's and St Michael's Pre-schools, Harvey House was re-opened as the Junior School and air raid shelters were built on the banks of the School Oval, which was re-named Hordern Oval.
The new Headmaster, Mr Gethyn Hewen, opened the War Memorial Hall, the Rose Bay Boatshed and the Prep School ("Dickins House") in Kent Road, Rose Bay. Northcott and Street Houses were established in 1957.
Mark Bishop was appointed Headmaster in 1963. During this decade, the first issue of the Old Cranbrookian Magazine was published and the first stage of the Mansfield Building was opened. Woodward House was founded in 1968.
A busy decade under the continued leadership of Mark Bishop: the Senior School Building was built, the Junior School moved to the Perkins Building and boarders were relocated from 'Leura' to Harvey House. Hone House was established in 1970.
Dr Bruce Carter began his leadership of the School in 1985 and the Student Council was formed. The Furber Building was opened and the first issue of The Chronicle was published. The iconic dark blue blazer was introduced as a regular part of the uniform. Cutler House was established in 1980.
Under the continued direction of Dr Bruce Carter, the Furber Building was completed and the Perkins Building was converted to a music centre. Born in the Hour of Victory was published by David Thomas and Mark McAndrew, chronicling the life and times of Cranbrook School's first 75 years. Perkins House was established in 1994.
Under the direction of new Headmaster Mr Jeremy Madin, Cranbrook acquired the Rose Bay Bowling Club, the Prep School became known as the Cranbrook Junior School and construction began on the Kent Road campus. The Carter Centre was opened and the Felton Room was renovated as a staff common room.
In addition, Cranbrook built the Justin McDonald Stand (2004), refurbished the heritage-listed Rotunda (renamed the John Saunders Pavilion in 2006) and developed plans for the new Junior School campus at Rose Bay.
Current Headmaster since 2012, Mr Nicholas Sampson, makes his mark by purchasing land at Wolgan Valley and launching the Senior School Masterplan. The new Junior School campus opened its gates in 2012.