William Windham, Senior, FRS (1717 – 30 October 1761) was an English landowner, a member of an ancient (Norfolk) family.
The son of Ashe Windham and Elizabeth Dobyns, he made an extensive Grand Tour of Europe in his youth, accompanied by his tutor, Benjamin Stillingfleet; the pair left England in 1737. During 1740–1741, Windham and Stillingfleet were members of a circle of British expatriates known as The Common Room. The circle lived in Geneva, and amused themselves with amateur theatricals (an unusual sight in that Calvinist city). Other notable members of the group included Richard Aldworth, the 7th Earl of Haddington and his brother, and Lord Hervey. In June 1741, several members of the circle, including Windham, joined Richard Pococke in making an expedition to Chamonix. They appear to have been the first recorded travellers for pleasure in the region, scaling Montenvers with the aid of local guides and giving the name of "Mer de Glace" to the glacier they subsequently examined. Stillingfleet and Windham overwintered there, and returned to England in July 1742.
Returning to England, he became a friend of the actor David Garrick. In 1744, he published a pamphlet, Letter from an English gentleman ... giving an account of a journey to the glacieres or ice alps of Savoy, describing his experiences and observations there. This piece of work, in addition to a talent for mathematics, secured his admission to the Royal Society on 26 April 1744.
He married late, having had an illegitimate daughter, Elizabeth, by his first mistress, Mary Morgan, and marrying his second, the widow Sarah Lukin, on 13 February 1750 shortly before the birth of his son, William, on 3 May 1750. The year before, Windham's father Ashe died, and he inherited the family seat of Felbrigg Hall.