Leinster was a member of the Irish House of Commons for Athy from 1741 before succeeding his father as 20th Earl of Kildare in 1744. He was sworn of the Irish Privy Council in 1746 and in 1747, on the occasion of his marriage, he was created Viscount Leinster, of Taplow in the County of Buckingham, in the Peerage of Great Britain, and took his seat in the British House of Lords that same year. From 1749 to 1755 he was one of the leaders of the Popular Party in Ireland, and served as the country's Master-General of the Ordnance between 1758 and 1766, becoming Colonel of the Royal Irish Artillery in 1760. He was promoted to Major-General in 1761 and to Lieutenant-General in 1770. In 1761 Lord Kildare was created Earl of Offaly and Marquess of Kildare in the Peerage of Ireland and in 1766 he was further honoured when he was made Duke of Leinster, becoming by this time the Premier Duke, Marquess and Earl in the Peerage of Ireland.
Lady Emily Mary FitzGerald, married Charles Coote, 1st Earl of Bellomont and Baron Coote of Coolony. The couple had five children, one son who died young and four daughters. Emily died in Penzance, Cornwall in 1818 after a lingering illness.
Lady Henrietta FitzGerald.
Lady Caroline FitzGerald.
Charles FitzGerald, 1st Baron Lecale.
Lady Charlotte Mary Gertrude FitzGerald, married Joseph Strutt and was made first Baroness Rayleigh. Had issue.
Lady Louisa Bridget FitzGerald.
Lord Henry FitzGerald, general; married Charlotte Boyle and had issue, who take by inheritance the surname "de Ros".
Lady Sophia Sarah Mary FitzGerald.
Lord Edward FitzGerald.
Lord Robert Stephen FitzGerald, a diplomat; married Sophia Charlotte Fielding and had issue.
Lord George Simon FitzGerald. Recognized as son of Lord Kildare, but was in fact the biological son of his brother's tutor, William Ogilvie.
Leinster died at Leinster House, Dublin, in November 1773, aged 51, and was buried in the city's Christ Church Cathedral. He was succeeded by his second son, William, Marquess of Kildare. The Duchess of Leinster caused a minor sensation by marrying her lover William Ogilvie in 1774, but continued to be known as The Dowager Duchess of Leinster. She had a further three children by him. She died in London in March 1814, aged 82.
Popular culture
Leinster appears in the 1999 BBC television series Aristocrats played by Ben Daniels.