[1] William Sinclair received a charter from James III of Scotland that was dated 7 December 1476 for the Earldom of Caithness to be held by him and his heirs.

The earl is party to a charter for the lands of Caithness that was executed at Castle Sinclair Girnigoe and dated 14 March 1496.

There is a remission cited as being made by George Hepburn who was Bishop of the Isles from 1510 to 1513, in favour of William, Earl of Caithness for all "Murders and crimes", which according to Roland Saint-Clair must mean incidents of disputed administration, feuds and property quarrels, and not personal felony.

[3] He also accompanied James IV of Scotland at the Battle of Flodden against the English army on 9 September 1513, where both of them were killed.

[1][2][4] He married Margaret Keith, daughter of Sir William Keith, 4th of Inverugie,[1][2] who also resided at Ackergill Tower which was a very strong keep at the most inland part of Sinclairs Bay, about two miles north of Castle Sinclair Girnigoe[3] They had the following children: Caithness may have had another child, Margaret, according to some sources.