Leinster was the son of Robert FitzGerald, 19th Earl of Kildare, and Lady Mary, daughter of William O'Brien, 3rd Earl of Inchiquin.^{[citation needed]}

Leinster was a member of the Irish House of Commons for Athy from 1741 before succeeding his father as 20th Earl of Kildare in 1743.^[1] He was sworn of the Irish Privy Council in 1746^[2] and in 1747, on the occasion of his marriage (see below), 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.^{[citation needed]} From 1749 to 1755 he was one of the leaders of the Popular Party in Ireland,^{[citation needed]} 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.^{[citation needed]}

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.^[3]

Leinster married the 15-year-old Lady Emily Lennox (6 October 1731 – London, 27 March 1814), daughter of Charles Lennox, 2nd Duke of Richmond and one of the famous Lennox Sisters, in London on 7 February 1747. She was a great-granddaughter of King Charles II and was therefore a distant fifth cousin of King George III (both of them were descended from King James VI and I). The couple had nineteen children:

• George FitzGerald, Earl of Offaly (15 January 1748 – Richmond House, 26 September 1765).
• William FitzGerald, 2nd Duke of Leinster (Arlington Place, Piccadilly, London, 12 March 1749 – Carton House, Kildare, 20 October 1804).
• Lady Caroline FitzGerald (1750 – 1754).
• Lady Emily Mary FitzGerald (15 March 1751 – 8 April 1818), married Charles Coote, 1st Earl of Bellomont and Baron Coote of Coolony. The couple had five children, one son who died young (born 1776) and four daughters. Emily died in Penzance, Cornwall in 1818 after a lingering illness.
• Lady Henrietta FitzGerald (1753 – 1763).
• Lady Caroline FitzGerald (born and died 1755).
• Charles FitzGerald, 1st Baron Lecale (30 June 1756 – 30 June 1810).
• Lady Charlotte Mary Gertrude FitzGerald (29 May 1758 – 13 September 1836), married Joseph Strutt and was made first Baroness Rayleigh. Had issue.
• Lady Louisa Bridget FitzGerald (1760 – 1765).
• Lord Henry FitzGerald (30 July 1761 – 8 July 1829), general; married Charlotte Boyle and had issue, who take by inheritance the surname "de Ros".
• Lady Sophia Sarah Mary FitzGerald (1762 – 21 March 1845).
• Lord Edward FitzGerald (15 October 1763 – 4 June 1798), revolutionary and prominent leader of the United Irishmen during the 1798 rising.
• Lord Robert Stephen FitzGerald (1765 – 2 January 1833), a diplomat; married Sophia Charlotte Fielding and had issue.
• Lord Gerald FitzGerald (January 1766 – 1788). Drowned, went down with the ship in which he was serving.
• Lord Augustus FitzGerald (1767 – 1771).
• Lady Fanny FitzGerald (1770 – 1775).
• Lady Lucy Anne FitzGerald (1771 – 1851), who took part in the Irish Rebellion of 1798. She was married to Admiral Sir Thomas Foley, who served with distinction under Lord Nelson. They had no children.
• Lady Louisa FitzGerald (1772 – 1776).
• Lord George Simon FitzGerald (16 April 1773 – 1783). Recognized as a 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 (but eldest surviving) 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.^{[citation needed]}

In 1999, Irish Screen, BBC America and WGBH produced Aristocrats, a six-part limited television series based on the lives of Emily Lennox and her sisters.^[4] Leinster was portrayed by Ben Daniels.

• Pedigree of the 1st Duke of Leinster