A son of Robert Sidney, 2nd Earl of Leicester, and the great-nephew of Sir Philip Sidney, he is thought to have been born at Penshurst Place in Kent. All of his life Sydney had been consistent in his support of liberty. He had served in the New Model Army, though he opposed the decision to execute Charles I. He was for a time the lover of Lucy Walter, later the mistress of Charles, Prince of Wales. In the end Oliver Cromwell's absolutism was little better for Sydney than that previously practiced by the king. His dismissal of the Long Parliament in 1653 was the act of a Caesarian dictator, subverting the republic and the constitution. In retirement Sydney was bold enough to outrage the Lord Protector by putting on a performance of Julius Caesar, with himself in the role of Brutus; and Brutus he was to remain.
A republican by deep conviction, he was abroad when the monarchy was restored in 1660, choosing to remain in exile for some years. While writing Court Maxims (1665-6) he was busy negotiating with the Dutch and French for support of a republican invasion of England. He was only to return in 1677, almost immediately becoming involved in opposition to Stuart monarchial absolutism. When Charles dismissed his final Parliament in 1681, saying he would have no more, Sydney united with Shaftesbury and others in plotting against the perceived royal tyranny, of a 'force without authority.' Sydney was later to be implicated in the Rye House Plot, a scheme to assassinate Charles and his brother, though on the evidence of only one witness.
Recognizing that a conviction rested on the testimony of two witnesses, Sidney pled the law and demanded a second witness be produced in court against him; at which juncture his own writings were offered into evidence, as, "false, seditious and traitorous libel". "An argument for the people", said the Solicitor General, "to rise up in arms against the King". In response Sydney said that it was easy to condemn him by quoting his words out of context "If you take the scripture to pieces you will make all the penmen of the scripture blasphemous; you may accuse David of saying there is no God and of the Apostles that they were drunk." But for the court, to write such was to act. The republican aristocrat was found guilty of treason and sentenced to death. Neither denying nor affirming the charge of treason for which he had been condemned, Sydney maintained republican faith to the end, declaring on the scaffold: "We live in an age that makes truth pass for treason."
For Sydney absolute monarchy, in the form practiced by Charles II, was a great political evil. His Discourses Concerning Government (the text for which Sydney lost his life) was written during the Exclusion Crisis, as a response to Robert Filmer's Patriarcia, a defence of divine right monarchy, first published in 1680. Sydney was appalled that a free-born Englishman could ever have compiled such a work, a defence of despotism. It was Filmer's business, he wrote, "to overthrow liberty and truth." Patriarchial government was not 'God's will', as Filmer and others contended, because the "Civil powers are purely human ordinances."
In countering the Hobbesian argument that the coercive power of the monarchy was necessary to prevent the return of the Civil Wars, Sydney invoked Tacitus, the Roman historian, saying that the pax Romana, the Imperial peace, was the 'peace of death.' Rebellion may have dangerous consequences but
They who are already fallen into all that is odious, and shameful and miserable, cannot justify fear... Let the dangers never be so great, there is the possibility of safety while men have life, hands, arms and courage to use them but that people must surely perish who tamely suffer themselves to be oppressed.