The school moved to its present premises on Kent Road in Southsea in 1885, when the building was opened by Princess Louise.

Dovercourt, the house built and lived in by the Southsea architect Thomas Ellis Owen, was acquired for the Junior School in 1927.

During World War II the school was evacuated to two country houses in Hampshire, Hinton Ampner (Junior Pupils) and Adhurst St Mary (Senior Pupils), and became a boarding school for six years.

[4] After the announced closure of the nearby St John's College in May 2022, it was announced in June 2022 that the long-running St John's Politics Society would be transferring to Portsmouth High School and rebranding itself as the "Portsmouth Politics Society".

Speakers over the years have included political big names such as former Prime Minister Margaret Thatcher, former Prime Minister Harold Wilson, Tony Benn, former Archbishop of Canterbury Rowan Williams, Douglas Hurd, former leader of UKIP and Reform UK Nigel Farage, former Foreign Secretary Jack Straw, former leader of the Green Party of England and Wales Caroline Lucas, former Prime Minister and former Home Secretary Theresa May, former Lord Chief Justice Lord Judge, former Director of Liberty Shami Chakrabarti, US Ambassador to the UK Matthew Barzun and former President of the Supreme Court Lord Neuberger.