Towards the end of the 18th century the owner of Forton Mill sold an adjacent parcel of land to the Board of Ordnance, to serve as the site for an Army General Hospital. Forton Military Hospital was begun in 1797; by 1800, however, soldiers were being treated at the nearby Naval hospital at Haslar, so the decision was taken to alter the proposed hospital buildings to serve as a barracks instead. The Barracks opened in 1807, consisting of four tall pavilions connected by arcades. The pavilions faced the main entrance gate across a sizeable parade ground, believed for a time to have been the largest in the country. Over the next forty years the barracks housed a succession of different regiments. The pavilions were capable of accommodating 832 men plus 48 sergeants; however by the 1840s one pavilion was still serving as a hospital, another had had its ground floor converted into the officers' mess and elsewhere space was taken up by school rooms, workshops and stores. At this time the Portsmouth Division of the Royal Marines resided in Old Portsmouth in premises which had formerly been a 17th-century brewery and cooperage: Clarence Barracks. Fast outgrowing their accommodation there, an arrangement was reached whereby the Board of Ordnance exchanged Forton Barracks for Clarence, allowing the Marines to move into Forton. To accommodate a full Royal Marine division, the barracks needed to expand: Captain Henry James R.E. oversaw the project, which included provision of a large officers' mess, along with additional officers' accommodation, to the east of the parade ground and a sergeants' mess west of the pavilions. The officers' mess included space upstairs for 80 diners plus a 30-piece orchestra, with a library and breakfast room provided downstairs along with kitchens to the rear. James also built two long single-storey blocks either side of the main gate, which he expanded with the addition of a clock tower, and he provided various buildings for stores and other amenities behind the main barrack blocks. The Royal Marines took possession of the barracks in 1848. Shortly afterwards they were retitled the Royal Marine Light Infantry. In 1858 the mill and millpond were purchased by the Admiralty; the millpond was drained became part of the site. By 1862 additional barrack blocks had been built between the old pavilions so as to accommodate the full complement of over 1000 men; further expansion, with the construction of married quarters for officers, took place in the 1890s. A 400-seater theatre was built within the site in 1893. In 1922 defence cuts meant that Portsmouth Division of the Royal Marine Light Infantry and the Royal Marine Artillery were amalgamated. The newly created Portsmouth Division of the Royal Marines moved to Eastney Barracks in August 1923 leaving the site empty. In 1927 the Barracks were recommissioned as HMS St. Vincent creating a Royal Naval training establishment for boy seamen and juniors. It continued as a shore establishment with the Royal Navy until 1969.
