Originally destined for a naval career, Chelmsford served with the Royal Navy during the Napoleonic Wars. On the death of his only surviving brother, he left the navy to study law. He proceeded to enter at Gray's Inn in 1813, and was called on the 18th of November 1818. He joined the home circuit, and soon got into good practice at the Surrey sessions, while he also made a fortunate purchase in buying the right to appear in the old palace court (see Lord Steward). In 1824 he distinguished himself by his defence of Joseph Hunt (the Elstree Murders) when on his trial at Hertford with John Thurtell for the murder of William Weare; and eight years later at Chelmsford assizes he won a hard-fought action in an ejectment case after three trials, to which he attributed so much of his subsequent success that when he was raised to the peerage he assumed the title Lord Chelmsford.
In 1834 he was made King's Counsel, and in 1835 was briefed in the Dublin election inquiry which unseated Daniel O'Connell. In 1840 he was elected M.P. for Woodstock. In 1844 he became Solicitor-General, but having ceased to enjoy the favor of the Duke of Marlborough, lost his seat for Woodstock and had to find another at Abingdon. In 1845 he became Attorney-General, holding the post until the fall of the Peel government in 1846.
On Lord Derby coming into office for the second time in 1858, Thesiger was raised straight from the bar to the lord chancellorship (as were Lord Brougham, Lord Selborne and Lord Haisbury). He served as Lord Chancellor again in Derby's 1866-67 government. In 1868 Lord Derby retired, and his successor, Benjamin Disraeli, wanted Lord Cairns as lord chancellor. After some upleasantness between Disraeli and Chelmsford, the latter resigned.
Lord Chelmsford had married in 1822 Anna Maria Tinling. He left four sons and three daughters, of whom the eldest, Frederick Augustus, 2nd Baron Chelmsford (1827 - 1905), earned distinction as a soldier, while the third, Alfred Henry Thesiger (1838-1880) was made a Lord Justice of Appeal and a privy councillor in 1877, at the early age of thirty-nine, but died only three years later.
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|width="30%" align="center"|Preceded by:
New Creation
|width="40%" align="center"|Baron Chelmsford
|width="30%" align="center" rowspan="2"|Followed by:
Frederick Augustus Thesgier
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