The son of a wealthy landowner, Eliot was first elected to Parliament in 1614 and was knighted four years later. In 1622 his friend George Villiers, Earl (later Duke) of Buckingham (Buckingham, George Villiers, 1st Duke of), the royal favourite, appointed him vice admiral of Devon. In 1623 Eliot succeeded in arresting a well-known pirate, Captain John Nutt, who thereupon used his political connections to have Eliot imprisoned for bribery. Six months later Buckingham stepped in to obtain his release.

Elected to the Parliament of 1624, Eliot won a reputation as an orator for his addresses defending freedom of speech for members of the House of Commons. He lost confidence in Buckingham after witnessing the mistakes and extravagances of Buckingham's foreign policy, and in the Parliament of 1626 he helped manage impeachment proceedings against the duke.

Charles I thereupon saved Buckingham by imprisoning Eliot (May 11–19, 1626) and dissolving Parliament. Eliot was suspended from his vice admiralty, and in June 1627 he was thrown into prison along with 74 other prominent gentry for refusing to contribute a forced loan to the crown.

Released in January 1628, he became the opposition leader in the Parliament that convened in March. There he made speeches against arbitrary taxation and in favour of preservation of English Protestantism from what he considered the Roman Catholic leanings of the archbishops. With Edward Coke and Peter Wentworth, he urged passage of the Petition of Right. In the Parliament of 1629, Eliot went further by drawing up three resolutions that vigorously condemned Charles's religious policy and his customs levies. Although Charles ordered Parliament to adjourn, Eliot had the speaker of the House of Commons held down in his chair until the resolutions were passed. The king had Eliot and eight others arrested. Imprisoned in the Tower of London, Eliot wrote several books on politics and ethics, including An Apology for Socrates and The Monarchie of Man. But his close confinement undermined his health; contracting tuberculosis, he died in the Tower.