In 1704 Mr. Guy was asked to become the governor of St. Thomas's Hospital, partly because he was a prominent and able citizen, and partly because he might thus become interested and give some money. Mr. Guy accepted the office, and soon built three new wards at a cost of £1,000, and provided the hospital with £100 a year for the benefit of its poor. When patients left the hospital they were often unfit for work, and this money would provide food for them for a time. He had given already to the steward money and clothes for such cases of need. He also built, in 1724, a new entrance to St. Thomas's Hospital, improved the front, and erected two large brick houses, these works costing him £3,000. Mr. Guy seems to have given constantly from his youth, and always with good sense in his gifts. He was growing old. He probably had meditated long and carefully as to what use he should put his wealth. Highmore, in his "History of the Public Charities of London," tells this rather improbable story: "For the application of this fortune to charitable uses the public are indebted to a trifling circumstance. He employed a female servant whom he had agreed to marry. Some days previous to the intended ceremony he had ordered the pavement before his door to be mended up to a particular stone which he had marked, and then left his house on business. "The servant, in his absence, looking at the workmen, saw a broken stone beyond this mark which they had not repaired; and on pointing to it with that design, they acquainted her that Mr. Guy had not ordered them to go so far. She, however, directed it to be done, adding, with the security incidental to her expectation of soon becoming his wife, 'Tell him I bade you, and he will not be angry.' But she soon learnt how fatal it is for one in a dependent position to exceed the limits of his or her authority; for her master, on his return, was angered that they had gone beyond his orders, renounced his engagement to his servant, and devoted his ample fortune to public charity." In 1721, when Mr. Guy was seventy-six years of age, he leased a large piece of ground of St. Thomas's Hospital for a thousand years at £30 a year, to erect upon it a great hospital for incurables; "to receive and entertain therein four hundred poor persons, or upwards, laboring under any distempers, infirmities, or disorders, thought capable of relief by physic or surgery; but who, by reason of the small hopes there may be of their cure, or the length of time which for that purpose may be required or thought necessary, are or may be adjudged or called incurable, and as such not proper subjects to be received into or continued in the present hospital, in and by which no provision has been made for distempers deemed or called incurable."