St. Leonard's Hill is situated in the parish of Clewer, on the edge of Windsor. The name derives from the chapel of St. Leonard of Losfield in Windsor Forest, established in the reign of King Edward III. The grounds are thought to have been the site of Roman occupation - possibly a temple - due to various artefacts brought to light there: numerous coins dating from the time of the Emperor Vespasian and the Lower Empire, spear-heads, arrows, pieces of 'trumpet' and fragments of ancient earthenware. However, the famous brazen lamp unearthed from beneath a stone, under which it had evidently been hidden, is now known to be medieval in date and probably came from the later chapel. It was presented by Sir Henry Sloane to the Society of Antiquaries, and has since been used as their logo.

The house seems to have started its life around 1700 as a 'hunting lodge,' presumably for the Royals in the surrounding forest. Although, considering its proximity to the castle, it was almost certainly only occupied for lunches or other entertainments. In the late 1750s, the Secretary of State, William Pitt (later Earl of Chatham), used to retire here for a little peace and contemplation during his direction of the Seven Years' War against France and the Austrian Empire.

In 1771, the house was purchased by the the celebrated beauty, the widowed Maria, Countess Waldegrave. She was the daughter of Edward Walpole and had grown up at nearby Frogmore House. Her sister and brother-in-law, the Dean of St. George's Chapel, already lived in the town. The income from Maria's late husband's appointments had evaporated at his death, but she continued to spent lavishly. She employed Thomas Sandby to extend her new house by two Adam-influenced rooms and called it 'Forest Lodge'. Luckily for Maria, the gentlemen at Court all vied for her attention and she soon found a new husband in the King's younger brother, Prince William, Duke of Gloucester, to whom she had bee introduced by her uncle, Horace Walpole. However, not only was Maria of low birth, but she was illegitimate, and George III did not approve. The Duke and Duchess were banished from Court and had survive on a meagre income. They withdrew to the lodge, which was renamed 'Gloucester Lodge' and expanded the estate with the purchase of the 'Hermitage' next door. By 1776, however, they were obliged to move abroad and the lodge was rented out to Henry Grenville. He added a fashionable temple and a grotto in the grounds.

A Mr. MacNamara bought the house in 1781, but sold it on, within the year, to the 3rd Earl Harcourt, who changed its name to 'St. Leonard's Hill'. He was a prominent Field Marshal in the British Army and friend of the Royal family. Locally, he was appointed Deputy Keeper of the Great Park and Deputy Governor of the Castle, and was a great benefactor to Clewer. When he died in 1833, the estate passed to his nephew, Edward Vernon (Harcourt), Archbishop of York; but the Earl's widowed sister, Sophie, the Marquise d'Harcourt, was allowed to live there for the next thirteen years. She had fallen in love with and married her distant cousin, the Marquis, when he had come to stay with the family after escaping the French Revolution. They had previously lived at the estate's dower house, St. Leonard's Dale.

After the death of the Marquise, the house was rented out to a number of occupants, including, briefly, the Earl of Derby, before he moved on to Coworth Park. By this time, the house was a very elegant residence standing in 261 acres, including pleasure grounds overlooking one of the most spectacular views in the county. Its entrance had a large portico leading to as stone-paved hall, the width of the building, with a fine stone staircase. It featured lofty dining and drawing rooms, over 40ft long with richly decorated ceilings. From the latter, a colonnade led to a 60ft conservatory and an adjoining 40ft 'Gothic' (revival) room with stained glass windows.

In 1872, this fine building was purchased by Francis Tress Barry, an English copper mining millionaire from Portugal, who had just been appointed Consul-General for Ecuador in England and needed a grand estate to reflect his position. He immediately employed Charles Howell to transform the place into a fashionable nouveau-riche-style French chateau, with steep mansarde roofs and an Italianate tower. The grey ragstone frontage, with expensive stone dressings, led into a magnificent octagonal hall. Beneath a circular glazed dome, Doric columns and a double staircase, supported a gallery, decorated with classic mythological frescoes above Mexican onyx panels, and featuring an organ for Lady Barry to play. Beyond the hall, in Lady Waldegrave's part of the house,  were the grand reception rooms: a dining room, small drawing room and Georgian salon with Adam-style ceilings. Then, through red lacquered mahogany doors, were the winter gardens, larger than the previous conservatory. Further ground-floor rooms included a massive billiard room, a smoking room, card room, gun room, study and a strong room, complete with iron door. Upstairs, the west wing featured six suites, including a Japanese suite with two octagonal rooms and a nursery suite of fifteen rooms. Although hot water was piped to the first floor, there were only two baths and two toilets. There was a late Georgian central heating system, but fireplaces were still widely used. Two lifts were also installed, one for service to the dining room and one for luggage to the top of the house. They would have been hydraulic or counter-balanced, as there was never electricity in the building. All lighting was gas and, as well as two wood-burning stoves, there was a gas-fired one. Below stairs, there were numerous rooms for a staff of thirty. There was also stabling for thirty horses, but motor cars were never allowed in the grounds.

Francis Barry was made a Portuguese baron in 1876 and, later an English baronet. He was good friends with the Prince of Wales, who occupied St. Leonard's Hill for the whole of Ascot Week in 1881. He often attended shooting parties there, even after he became King Edward VII. Barry was a great patron to Windsor, especially the Royal Infirmary, and donated the Alexandra Gardens to the town. After he died in 1907, Lady Barry carried on at the house, but, during World War One, the World changed. There were not enough staff to run such a large house and attempts were twice made to sell it. However, apart from the lack of technological advancements installed, the sewerage system was still based on cesspits. It was unsuitable for use as an institution and after Lady Barry's death in 1926, St. Leonard's Hill was pulled down by her son, Sir Edward, who lived at the very different Ockwells and hated the place. Only dynamite had any impression, but, even then, the shell remained for decades. The grounds were slowly built upon during the 20s and 30s, while the land around the ruins was purchased by Mr. Reg Try in 1941. He planted camellias and, for many years, allowed the local people to hold events there and appreciate the view.

St. Leonard's Hill no longer stands. Only a few minor ruins remain inaccessible on private land.