He was educated abroad and at the University of Cambridge.
He quickly gained the favour of King Henry VIII (1509–1547), and in 1537 was appointed president of a commission of enquiry into the condition of Ireland. In the course of this work, he obtained much useful knowledge of the country. In 1539, he was knighted and appointed Sheriff of Kent. In 1540 he was appointed lord deputy of Ireland. His first task was to repress disorder, and he at once proceeded with severity against the Kavanaghs, permitting them, however, to retain their lands, on their accepting feudal tenure on the English model. By a similar policy he exacted obedience from the O'Mores, the O'Tooles and the O'Conors in Leix and Offaly; and having conciliated the O'Briens in the west and the earl of Desmond in the south, the lord deputy carried an act in the Irish parliament in Dublin conferring the title of king of Ireland on Henry VIII and his heirs. Conn O'Neill, who in the north had remained sullenly hostile, was brought to submission by vigorous measures. For the most part, however, St Leger's policy was one of moderation and conciliation-rather more so, indeed, than Henry VIII approved. He recommended The O'Brien, when he gave token of a submissive disposition, for the title of earl of Thomond; O'Neill was created earl of Tyrone; and administrative council was instituted in the province of Munster; and in 1544 a levy of Irish soldiers was raised for service in Henry VIII's wars. St Leger's personal influence was proved by an outbreak of disturbance when he visited England in 1544, and the prompt restoration of order on his return some months later. St Leger retained his office under Edward VI, and again effectually quelled attempts at rebellion by the O'Conors and O'Byrnes. From 1548 to 1550 he was in England. He returned charged with the duty of introducing the reformed liturgy into Ireland. His conciliatory methods brought upon him the accusation that he lacked zeal in the cause, and led to his recall in the summer of 1551. After the accession of Mary he was again appointed lord deputy in October 1553, but in consequence of a charge against him of keeping false accounts he was recalled for the third time in 1556. While the accusation was still under investigation, he died on the 16th of March 1559.
A biography of Sir Anthony St Leger will be found in Athenae Cantabrigienses, by Charles Henry Cooper and Thompson Cooper (Cambridge, 1858).
According to the Irish Genealogical Office, Kildare Street, Dublin, Sir Anthony St Leger KG held office as the King's Deputy (Lord Deputy) in Ireland for five not three terms as commonly held. His terms of office were as follows :
1st term: 7 July 1540 to 10 February 1544 2nd term: 3 July 1544 to 1 April 1546 3rd term: 7 November 1546 to 21 May 1548 4th term: 4 August 1550 to 23 May 1551 5th term: 1 September 1553 to 26 May 1556.
St Leger seems to have been a quarrelsome and unpopular man; certainly, he was on very bad terms with other leading figures in the Dublin administration, particularly John Alan, Lord Chancellor of Ireland, and George Browne, Archbishop of Dublin. Following complaints by St Leger, Alan was removed from office and although he was later reinstated, the two men found it impossible to work together. Archbishop Browne accused St. Leger of treasonable words, giving Alan as his source, but the charge came to nothing when Alan, surprisingly, refused to confirm the report. When St Leger gave high military command to James Butler, 9th Earl of Ormond, he was accused by allies of Alan's of deliberately doing this to endanger Ormond's life. Ormond's mysterious death from poisoning in London in 1546, with 16 of his household, when he went to dinner in the Ely Palace was, at the least, extremely convenient for St Leger, although there is no direct evidence that he was responsible for it (mainly because there was no investigation, despite Ormond's wealth and social standing).
St Leger married Agnes Warham, daughter of Sir Hugh Warham of Croydon, and niece of William Warham, Archbishop of Canterbury. By Agnes, he had at least five sons and two daughters.
He was an English soldier.
He was an English-born judge in Ireland who held the office of Master of the Rolls in Ireland.
He predeceased his father, albeit having left a son Sir Warham St Leger (d. 1600), whose own son was William St Leger (d. 1642), President of Munster.