Ellis was the second but only surviving son of the Most Reverend Welbore Ellis, Bishop of Kildare and Bishop of Meath. He was educated at Westminster School from 1727 to 1732 and then entered Christ Church, Oxford.

In 1741, he was elected Member of Parliament (MP) for Cricklade, then moved to Weymouth and Melcombe Regis (1747–1761), Aylesbury (1761–1768), Petersfield (1768–1774), Weymouth and Melcombe Regis (1774–1790) and Petersfield (1791–1794).^[1]

In 1762, he succeeded Charles Townshend as Secretary at War, and in 1763, he proposed the appropriation of twenty army regiments to the colonies of America. In Parliament, with many others, he opposed the reception of papers from the American Continental Congress. He became Treasurer of the Navy in 1777, then succeeded to the Colonial Secretaryship in 1782, which he held for a matter of months, before the American colonies were lost. In 1784, he became the longest-serving member of the House of Commons (having served for 43 years non-continuously), becoming the honorary Father of the House.

He was created Baron Mendip, of Mendip in the County of Somerset, in 1794 in recognition of his governmental service. The peerage was created with remainder to the three eldest sons of his sister Anne by her husband Henry Agar, of Gowran and Gowran Castle.

In 1738 he inherited a large fortune from his uncle, John Ellis and built Clifden House in Brentford.

He married firstly in 1747 Elizabeth, the daughter and heiress of Sir William Stanhope and secondly in 1765 Anne, the daughter of George Stanley of Paultons, Hampshire. Ellis nevertheless died childless in February 1802, aged 88, and was succeeded in the barony according to the special remainder by his great-nephew, Henry Welbore Agar, 2nd Viscount Clifden, who assumed the surname of Ellis two years later.

• Viscount Clifden

• "Archival material relating to Welbore Ellis, 1st Baron Mendip". UK National Archives.