William was the son of Sir Alexander de Windsor of Grayrigg, Westmorland, and of Elizabeth (died August 1349), his wife.

[c] He at once set to work to reduce the Dublin border clans, but in 1370 had to leave them in order to attempt the rescue of the Earl of Desmond, who had been taken prisoner by the O'Briens.

[d] To secure even partial order Windsor had been compelled to adopt measures of doubtful legality; at a parliament of 1369, failing to induce its members to promise new customs to the King, he extorted from the prelates, who met separately, a grant for three years, and afterwards had enrolment made in the chancery records that they were given in perpetuity to the Crown.

The colonists appealed to Edward III, and, in answer to their petition, the King, on 10 September 1371, forbade Windsor, who had returned to England in March, to levy the sums for which he had exacted grants, ordered the enrolment to be erased, and on 20 October formally rebuked him for his extortions, which he bade him make good.

[j] In 1374, on the refusal of a parliament at Kilkenny to make a grant at Dagworth's request, Windsor issued writs bidding clergy and laity to elect representatives, finance them, and send them to England to consult Edward on an aid to be taken from Ireland.

The government sent help by sea to the garrison in the castle of Wicklow, but the council, meeting at Naas, forbade Windsor to move further south because it left the north in peril.

[l] In the same year Windsor was sent on the expedition to help the Duke of Brittany against France,[m] receiving large grants of land, most of which had been forfeited by Alice Perrers.