His brothers included Philip (1619–1698), who fought for Parliament during the Wars of the Three Kingdoms, and Algernon (1623–1683), a political theorist closely associated with John Locke who was executed in 1683 for his part in the Rye House Plot.

[4] Although Sydney had an illegitimate son, Henry Worthley, he never married; on 8 April 1704, he died of smallpox at his house in London and was buried a few days later in St James's Church, Piccadilly.

Although the war ended before he saw active service, he became friends with William of Orange; with the support of Sunderland, then Secretary of State for the Northern Department, in June 1679 he was appointed Envoy to the United Provinces, a position he held with marked success until 1681.

[6] In 1683, Algernon Sydney was executed for his part in the Rye House Plot, an alleged conspiracy to assassinate Charles II along with his brother James, and place Monmouth on the throne.

Sydney was thus able to balance his obligations as a loyal subject with acting as an informal contact point between William and his English supporters, as well as Sunderland, whom James appointed Lord President of the Council in 1685.

[10] On 4 April, he was created Baron Milton and Viscount Sydney, became a Privy Councillor and received a number of other offices, including Lord Lieutenant of Kent and Constable of Dover Castle.

Although his tenure was marked by conflict with the Irish Parliament and lasted less than a year, he retained William's confidence; in 1693, he was made Master-General of the Ordnance, then promoted Lieutenant-General and created Earl of Romney in 1694.

[5] Sydney's reputation has been subject to a variety of views; some contemporaries considered him lazy and superficial, while Tory satirist Jonathan Swift dismissed him as 'an idle, drunken, ignorant rake, without sense or honour'.