The 1st Duke of Leinster.

In 1747, on the occasion of his marriage to Lady Emily Lennox, daughter of the 2nd Duke of Richmond, he was in the Peerage of Great Britain created Viscount Leinster, of Taplow in the County of Buckingham, and took his seat at Westminster 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.

In 1761 Lord Kildare was created Marquess of Kildare and Earl of Offaly in the Peerage of Ireland, and in 1766 he was created Duke of Leinster, becoming by this time the Premier Duke, Marquess and Earl in the Peerage of Ireland. This followed from his marriage to Emily, who was descended from King Charles II and was therefore a cousin of King George III.

His eldest surviving son, William, Marquess of Kildare, succeeded as 2nd Duke of Leinster, his third son, Lord Charles FitzGerald, was created Baron Lecale in the Peerage of Ireland in 1800, and his fifth son was the Irish revolutionary Lord Edward FitzGerald.

Leinster died aged 51 at Leinster House in Dublin, and was buried in the city's Christ Church Cathedral.

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