In 1629 he entered Trinity College, Oxford, as a gentleman commoner. One of his tutors was the famous William Chillingworth. After several years spent travelling, and a period as a soldier in the Dutch army, he returned to England and lived quietly till 1646, when he was appointed to accompany King Charles I, who was being taken from Newcastle as a prisoner. Though republican in his ideas, Harrington won the king's regard and esteem, and accompanied him to the Isle of Wight. He aroused the suspicion of the parliamentarians and was dismissed: it is said that he was punished for declining to swear to refuse assistance to the king should he attempt to escape.
After Charles's death Harrington devoted his time to the composition of his Oceana, a work which pleased no one. By order of Oliver Cromwell, it was seized when passing through the press. Harrington, however managed to secure the favour of the Protector's favourite daughter, Mrs Claypole; the work was restored to him, and appeared in 1656, dedicated to Cromwell. The views embodied in Oceana, particularly that bearing on vote by ballot and rotation of magistrates and legislators, Harrington and others (who in 1659 formed a club called the "Rota") endeavoured to push practically, but with no success.
In November 1661, by order of Charles II, Harrington was arrested on a charge of conspiracy, and was thrown into the Tower of London. Despite his repeated request no public trial took place, and when at length his sisters obtained a writ of habeas corpus he was secretly moved to St Nicholas Island off Plymouth. There his health gave way owing to his drinking guaiacum on medical advice, and his mind appeared to be affected. Careful treatment restored him to bodily vigour, but his mind never wholly recovered. Some time after his release he married--the date does is not known. Following his death, he was buried next to Sir Walter Raleigh in St Margaret's, Westminster.
Harrington's writings consist of the Oceana, and of papers, pamphlets, aphorisms, even treatises, in defence of the Oceana. His Works were edited with biography by John Toland in 1700; Toland's edition, with additions by Thomas Birch, appeared in 1747, and again in 1771. Oceana was reprinted by Henry Morley in 1887. See Dwight in Political Science Quarterly (March, 1887). Harrington has often been confused with his cousin Sir James Harrington, a member of the commission which tried Charles I, and afterwards excluded from the acts of pardon.