- For the MP for County Mayo, see Sir William Brabazon, 2nd Baronet.
After succeeding his father he was knighted on 20 Aug. 1534, and appointed vice-treasurer and general receiver of Ireland. In a letter from Chief-justice Aylmer to Lord Cromwell in August 1535 he is styled 'the man that prevented the total ruin and desolation of the kingdom.' In 1536 he prevented the ravages of O'Connor[clarification needed] in Carberry by burning several villages in Offaly and carrying away great spoil. In the same year he made so effective a speech in support of establishing the king's authority in opposition to that of the pope that he persuaded the parliament to pass tho bill for that purpose. As a result of this, many religious houses were in 1539 surrendered to the king.
For these and other services he was, on 1 October 1543, constituted lord justice of Ireland, and he was again appointed to the same office on 1 April 1546. In the same year he drove Patrick O'More and Brian O'Connor from Kildare. In April 1547 he was elected a member of the Privy Council of Ireland. In the spring of 1548 he assisted the lord deputy in subduing a sedition raised in Kildare by the sons of Viscount Baltinglass. He was a third time made lord justice on 2 Feb. 1549. In August 1550, with the aid of 8,000l. and 400 men from England, he subdued Charles Mac-Art-Cavenagh, who, after making submission and renouncing his name, received pardon.
Brabazon died on 9 July 1552 (as is proved by the inquisitions taken in the year of his death), not in 1548 as recorded on his tombstone. His heart was buried with his ancestors at Eastwell, and his body in the chancel of St. Catherine's Church, Dublin. By his wife Elizabeth, daughter and coheir to Nicholas Clifford of Holme, he left two sons and three daughters.