He married Jean or Janet Edmonstone, daughter of the Laird of that Ilk [of Edmonstone] in the Merse, by whom he had a son, Sir William Sinclar of Pentland, knight, his successor. On the 20th March 1617, William Sinclair and his son Sir William resigned the Baronies of Roslin and Pentland, and a new infeftment issued to Dame Anna Spotswood, spouse to the said Sir William, in liferent, of all and haill the lands and town of Pentland, and suchlike of the lands of Otislie, and the part of Roslin then occupied by Robert Park, and also for new infeftment to be made to the said Sir William and to his heirs-male; which failing to Robert Sinclair of Longformacus, etc.; which failing to the said Sir William's nearest heirs, bearing the surname and arms of Saintcler, of the Baronies of Roslin, Pentland, Morton, and Mortonhall. Sasine followed on the 1st February 1619, in the presence of Oliver Saintcler, brother-german [full brother] of Henry Sinclair of Whitekirk. Hay writes of him: "Sir William Sinclair was a lewd man. He kept a miller's daughter, with whom, it is alleged, he went to Ireland; yet I think the cause of his retreat was rather occasioned by the Presbyterians, who vexed him sadly because of his religion being Roman Catholic".