After visiting Scotland on a diplomatic mission in 1588, and France on a similar errand in 1593, he returned to the Netherlands in 1606, where he rendered distinguished service in the war for the next two years. He had been appointed governor of Flushing in 1588, and he spent much time there till 1603, when, on the accession of James I, he returned to England. James raised him at once to the peerage as Baron Sidney of Penshurst, and he was appointed chamberlain to the queen consort, Anne of Denmark. In 1605 he was created Viscount Lisle, and in 1618 Earl of Leicester, the latter title having become extinct in 1588 on the death of his uncle, whose property he had inherited.
Leicester was a man of taste and a patron of literature, whose cultured mode of life at his country seat, Penshurst, was celebrated in verse by Ben Jonson. It was at Penshurst that he died. He had been married twice; first to Barbara Gamage, a noted heiress and beauty, the daughter of John Gamage, a Glamorgan gentleman; and secondly to Sarah, daughter of William Blount, and widow of Sir Thomas Smythe. By his first wife he had a large family. His eldest son having died unmarried in 1613, Robert, the second son, succeeded to the earldom; Philippa, one of his daughters married Sir John Hobart, third son of Sir Henry Hobart, Lord Chief Justice of the Common Pleas; ancestor of the Earls of Buckinghamshire. Another daughter, Mary Wroth, was (like her father) a poet; Ben Jonson dedicated The Alchemist to her in 1612.
Though the brother of one of the most famous poets in the English language, it was not suspected that Robert Sidney had himself been a poet until the 1960s, when his working notebook emerged (in a 19th-century binding) through the dispersal of the Library of Warwick Castle. (Subsequent research showed it had been acquired in 1848 after passing through a number of sales beginning with the dispersal of the library at Penshurst in the early 19th century.) Sold again at Sotheby’s and acquired by the British Library in 1975, the autograph is, as its first editor P.J. Croft pointed out, 'the largest body of verse to have survived from the Elizabethan period in a text entirely set down by the poet himself'. Dating apparently from the latter half of the 1590s when Robert Sidney was governor of Flushing, the collection comprises 66 sonnets, songs, pastorals, elegies and slighter pieces, apparently structured as a kind of reply to Philip Sidney’s Astrophel and Stella. They show Robert Sidney as an advocate of the Neo-platonic philosophy of love and adept at a great variety of verse forms. The fact that several of the poems are based on identifiable tunes confirms his interest in music. While he cannot be placed in the first rank of Elizabethan poets, his poems are by no means negligible and of the greatest interest for the working methods and intellectual interests of the period.