Sir Philip, was born, as is supposed, at Penshurst in Kent, in the year 1544. His father was Sir Henry Sidney, an Irish gentleman, and his mother, Mary, the eldest daughter of John Dudley, Duke of Northumberland. He was sent, when very young, to Christ-Church College at Oxford, but left the university at seventeen to set out on his travels. After visiting France, Germany, Hungary, and Italy, he returned to England in 1575, and was next year sent by Queen Elizabeth, as her ambassador to Rudolph, the Emperor of Germany. On his return he visited Don John of Austria, governor of the Netherlands, by whom he was received with great respect. In 1579, when Queen Elizabeth seemed on the point of concluding her long projected marriage with the Duke of Anjou, Sir Philip wrote her a letter, in which he dissuaded her from the match with unusual elegance of expression, and great force of reasoning. About this time a quarrel with the Earl of Oxford occasioned his withdrawing from court; during which retirement he is supposed to have written his celebrated romance called Arcadia.

In 1585, after the Queen's treaty with the United States, he was made governor of Flushing and master of the horse. Here he distinguished himself so much both by his courage and his conduct, that his reputation rose to the highest pitch. He was named, it is said, by the republic of Poland as one of the competitors for that crown, and might even have been elected, had it not been for the interference of the queen. But his illustrious career was soon terminated; for being wounded at the battle of Zutphen, he was carried to Arnhem, where he died on the 15th of October 1586. His body was brought to London, and buried in St. Paul's cathedral. He is described, by the writers of that age, as the most perfect model of an accomplished gentleman that could be formed even by the wanton imagination of poetry or of fiction. Virtuous conduct, polite conversation, heroic valour, and elegant erudition, all concurred to render him the ornament and delight of the English court; and as the credit which he enjoyed with the queen and the Earl of Leicester was wholly employed in the encouragement of genius and literature, his praises have been transmitted with advantage to posterity. No person was so low as not to become an object of his humanity. After the battle of Zutphen, whilst he was lying on the field mangled with wounds, water was brought him to relieve his thirst; but observing a soldier in a like miserable condition, looking wistfully towards him, he resigned it to him, saying, "This man's necessity is still greater than mine." Besides his Arcadia, he wrote a discourse on Poetry, and some other pieces both in prose and verse, which have been published. There is a complete edition of his works in three volumes octavo, Lond. 1725.

Algernon, was the second son of Robert Earl of Leicester, and Dorothy, eldest daughter of the Earl of Northumberland. He was born in 1617. During the civil wars he took part against the king, and distinguished himself as a colonel in the army of the parliament. He was afterwards appointed one of King Charles's judges, but declined appearing in that court. During the usurpation of Cromwell, Sidney, who was a violent republican, retired to the country, and spent his time in writing those discourses on government which have been so deservedly celebrated. After the death of the Protector, he again took part in the public transactions of his country, and was abroad on an embassy to Denmark when Charles was restored. Upon this he retired to Hamburg, and afterwards to Frankfort, where he resided till 1677, when he returned to England and obtained from the king a pardon. It has been affirmed, but the story deserves no credit, that, during his residence abroad, Charles hired ruffians to assassinate him. After his return he made repeated attempts to procure a seat in parliament, but all of them proved unsuccessful. After the intention of the Commons to exclude the Duke of York from the throne had been defeated by the sudden dissolution of parliament, Sidney joined with eagerness the councils of Russel, Essex, and Monmouth, who had resolved to oppose the duke's succession by force of arms. Frequent meetings were held at London; whilst at the same time, a set of subordinate conspirators, who were not, however, admitted into their confidence, met and embraced the most desperate resolutions. Keiling, one of these men, discovered the whole conspiracy; and Algernon Sidney, together with his noble associates, was immediately thrown into prison, and no art was left unattempted to involve them in the guilt of the meanest conspirators.

Howard, an abandoned nobleman, without a single spark of virtue or honour, was the only witness against Sidney; but as the law required two, his discourses on government, found unpublished in his closet, were construed into treason, and declared equivalent to another witness. It was in vain for Sidney to plead that papers were no legal evidence; that it could not be proved they were written by him; and that if they were, they contained nothing treasonable. The defence was overruled; he was declared guilty, condemned, and executed on Towerhill, December 7, 1678. His attainder was reversed in the first year of William III.

He was a man of extraordinary courage, steady even to obstinacy, and of a sincere but rough and boisterous temper. Though he professed his belief in the Christian religion, he was an enemy to an established church, and even, according to Burnet, to every kind of public worship. In his principles he was a zealous republican. Government was always his favourite study; and his essays on that subject are a proof of the progress which he made. His Treatise on Government, and some other pieces, with memoirs of his life, were published by Thomas Holles, in 1763, in a quarto volume.