Aged 25, Leigh was elected High Steward of the University of Oxford and was made a Doctor of Civil Law.
At the same time, he was active at Stoneleigh: collecting art, furniture and books, he also made architectural plans for the house.
In 1766 and 1797 payments are recorded to Bedlam Hospital and John Munro or his son Thomas Munro, the mad-doctors who later attended to George III, and in 1774 an Inquisition of Insanity found that Leigh had been 'a Lunatick of unsound mind' over the previous five years and committed him to the guardianship of his sister, Hon.
A prayer written by his sister Mary in about 1775 survives: O Lord look down from Heaven, in much pity and compassion, upon thy afflicted servant, who is not able to now look up to thee, hear O most merciful Father my Prayers on his behalf, and preserve him from doing any harm to himself or to any other: be pleased to remove all frightfull imaginations far from him, and if it be the blessed will, O our God restore him to his reason and understanding, so will we all give thanks to thee for ever and ever.
Among the provisions, the will gave Leigh's scientific instruments and his library of about 1,000 books to his alma mater, Oriel College.