Appointed Vice Treasurer and General Receiver of Ireland in 1534, which office he retained until his death Lord Justice of Ireland in 1543, 1546 and 1550. In 1543 the seals changed, as the King became King of Ireland, not Lord of Ireland Appointed Vice Treasurer and General Receiver of Ireland in 1534, which office he retained until his death Lord Justice of Ireland in 1543, 1546 and 1550.

In 1543 the seals changed, as the King became King of Ireland, not Lord of Ireland Knighted and on 26th August 1534 appointed Vice Treasurer and General Receiver of Ireland which he held to his death on 7th of the Ides of July (viz. the 9th day of that month) 1552; which year, says Sir Richard Cox, "was unhappy, not only by the civil idssensions in Ulster, between the Earl of Tyrone and his son Shane O'Neil, and by the scarcity of provisions; but also by the death of Sir William Brabazon, who died in July, and was one of the most faithful men to the English interest that had appeared in Ireland from the conquest to that day".

Indeed, his capacity and abilities were so conspicuous, that in a letter, dated 21st August 1535, from the Lord Chief Justice Aylaner to the Lord Cromwell, Prime Minister to King Henry VIII he is styled "the Man that prevented the total ruin and desolation of the country, and is extolled by them as the Saver of the Kingdom." In 1536, with the Lord Chancellor Trimleston, he prevented the ravages of O'Connor in Carbury, by burning many villages in his Country of Offaley, and carrying away great preys. And in the Parliament held that year, there being much difficulty to get the acts for establishing the King's, and abolishing the Pope's Supremacy and Jurisdiction in Ireland, to pass both houses, he seconded the Speech made by Archbishop Browne so effectually, that the Houses were startled at it, and prevailed on at length to pass both the bills.

As a consequence hereof, in 1539, many abbots and priors, with their fraternities, treading in the steps of their brethren in England, made a voluntary surrender, by charters under their common seals, of their Abbies, with all their furniture and goods, to the King, to whom annual pensions for life were assigned out of the revenues which the King had reserved to himself; and the disposal of all the abbies in Ireland was given to his Majesty by parliament.

These surrenders were taken for his Majesty's use by the Chancellor Alien, the Vice-Treasurer Brabazon, and Robert Cowley, Master of the rolls, Commissioners for that purpose; who were also commissioned the next year (1540) to grant the said annual pensions to the monks of the suppressed abbies. These and other services to his Prince, rendered him worthy to be entrusted with the chief administration of affairs; and accordingly, 12th October, 1543, he was constituted Lord Justice of the Kingdom, and sworn 10th February following and to him new Seals for the use of the Chancery, Exchequer, and both the Benches, were sent upon the alteration of the King's style from Lord to King of Ireland. After which, at the King's command, who had declared war against Francis the French king, he sent his Majesty 700 Foot to Calais, under the command of Poer, Finglas and Scurlock, who did him great service at Bulloign.

On late April, 1546 he was again appointed Lord Justice of Ireland, and that year pursued Patrick 0'More and Brien 0 'Connor, who had entered the County of Kildare, and, among other damages, burned a great part of Athy, in the castle whereof he left a garrison, proclaimed them traitors, marched into Offaley, made a fort at Phlilipstown, destroyed their territories, and forced 0'Connel to fly into Connaught.

After King Edward's accession to the Crown, his Majesty, by letter to the Lord Deputy St. Leger, dated at Greenwich, 7th April 1547, directed, that in respect of his faithful and diligent service done to his father and himself, be should be one of his Privy Council. In which year, the Castle of Athlone, at his motion and instigation, being repaird and garrisoned by order of counceil, the charge therof was committed to him, which he most effectually performed, notwithstanding the great opposition of Dominick 0 Kelly and other powerful chiefs in Connaught.

In the Spring of 1548 he accompanied the Lord Deputy against Richard and Alexander, the younger sons of Thomas Viscount Baltinglas, then raising sedition in Kildare, which was suppressed in its very contrivance by their submission. On 2nd February, 1549, he was a third time made Lord Justice, being so elected by the Council, on the death of Sir Francis Bryan, after which he made a journey to Limerick, where Teige 0'Carrol submitted, was pardoned, and subscribed to certain articles, binding himself not to assume the title of President of Ormond, to pay to the Exchequer a yearly rent, and maintain a certain number of horse and foot for the King's service; by whose intercession also, Letters of Protection were granted to Mac-Murrough, 0'Kelly, and 0'Melaghlin. And receiving from England 80001, and 400 men, in August 15550, he pursued, with indefatigable labour, Charles MacArt Cavenagh, again in rebellion, declared him traitor, killed many of his followers, and burned his country, which obliged him to make a very formal submission, in Dublin, on the 4th of November following, to renounce the name of Mac-Murrough, and part with some of his usurped jurisdiction and estate.

In the Spring of 1548 he accompanied the Lord Deputy against Richard and Alexander, the younger sons of Thomas Viscount Baltinglas, then raising sedition in Kildare, which was suppressed in its very contrivance by their submission. On 2nd February, 1549, he was a third time made Lord Justice, being so elected by the Council, on the death of Sir Francis Bryan, after which he made a journey to Limerick, where Teige 0'Carrol submitted, was pardoned, and subscribed to certain articles, binding himself not to assume the title of President of Ormond, to pay to the Exchequer a yearly rent, and maintain a certain number of horse and foot for the King's service; by whose intercession also, Letters of Protection were granted to Mac-Murrough, 0'Kelly, and 0'Melaghlin. And receiving from England 80001, and 400 men, in August 15550, he pursued, with indefatigable labour, Charles MacArt Cavenagh, again in rebellion, declared him traitor, killed many of his followers, and burned his country, which obliged him to make a very formal submission, in Dublin, on the 4th of November following, to renounce the name of Mac-Murrough, and part with some of his usurped jurisdiction and estate.

And on the Graves-stone, under the Monument, is this circumscription:
Here Lieth the Body of Sir William Brabazon, Knt.
Who continued Treasurer in this Kingdom XXXII
Years, in which Time he was Lord Justice v several
Times; he was the first Englishman that planted in
Connaught, and wan the Castle of Athlone. He served
In the Reigne of King Henry the VIII, and King Edward
The VI. His Son, Sir Edward Brabazon, Knt. Lord Baron of
Ardee, purposeth to be entomed by his Father and Mother