Sadleir's first involvement with the office of arms at Dublin Castle was when he worked on an unpaid basis whilst an undergraduate at Trinity College, Dublin. He graduated in 1904, and was called to the bar in 1906.
By 1913 he was working on a daily basis at the office, whilst practising as a barrister. In 1915 he was appointed registrar of the Order of St Patrick by George Dames Burtchaell, Deputy Ulster King of Arms. In practice, Sadleir carried out most of the day-to-day work of Ulster's office.
In August 1921 Burtchaell was killed in a tram accident, and in September Sadleir was appointed Deputy to Neville Rodwell Wilkinson, Ulster King of Arms. As Wilkinson was almost always absent from Dublin, Sadleir performed most of the duties of the office.
In 1940 Wilkinson died, and the government of Ireland requested that no successor be appointed. For the next three years Sadleir was Acting King of Arms.
In 1943 the government of Ireland established the Genealogical Office which took over the records of the Office of Arms, while the title of Ulster King of Arms was merged with that of Norroy to become Norroy and Ulster King of Arms, a member of the College of Arms in London.
Sadleir continued to work for the Genealogical Office until 1944, clearing the large backlog of grants and confirmations of arms that had built up in Ulster's office. After leaving the GO, he continued his private genealogical practice. He maintained links with his former employer, however, remaining a trustee of the Heraldic Museum until his death.
Sadleir subsequently became librarian at King's Inns, Dublin, a post he held until his death.
- Royal Roots, Republican Inheritance - The Survival of the Office of Arms, Susan Hood, Dublin, 2002
- Sadleir Papers, Genealogical Office