In 1572, at the age of 18, he was elected to Parliament as a Member of Parliament for Shrewsbury and in the same year travelled to France as part of the embassy to negotiate a marriage between Elizabeth I and the Duc D'Alençon. He spent the next several years in mainland Europe, moving through Germany, Italy, Poland, the Kingdom of Hungary and Austria. On these travels, he met a number of prominent European intellectuals and politicians.
Returning to England in 1575, Sidney met Penelope Devereux (who would later marry Robert Rich, 1st Earl of Warwick). Although much younger, she would inspire his famous sonnet sequence of the 1580s, Astrophel and Stella. Her father, Walter Devereux, 1st Earl of Essex, was said to have planned to marry his daughter to Sidney, but he died in 1576 and this did not occur. In England, Sidney occupied himself with politics and art. He defended his father's administration of Ireland in a lengthy document.
More seriously, he quarrelled with Edward de Vere, 17th Earl of Oxford, probably because of Sidney's opposition to the French marriage of Elizabeth to the much younger Alençon, which de Vere championed. In the aftermath of this episode, Sidney challenged de Vere to a duel, which Elizabeth forbade. He then wrote a lengthy letter to the Queen detailing the foolishness of the French marriage. Characteristically, Elizabeth bristled at his presumption, and Sidney prudently retired from court.
During a 1577 diplomatic visit to Prague, Sidney secretly visited the exiled Jesuit priest Edmund Campion.
Sidney had returned to court by the middle of 1581 and in 1584 was MP for Kent. That same year Penelope Devereux was married, apparently against her will, to Lord Rich. Sidney was knighted in 1583. An early arrangement to marry Anne Cecil, daughter of Sir William Cecil and eventual wife of de Vere, had fallen through in 1571. In 1583, he married Frances, the 16-year-old daughter of Sir Francis Walsingham. In the same year, he made a visit to Oxford University with Giordano Bruno, the polymath known for his cosmological theories, such as affirming Copernicus at a time when many others did not, and speculating that the stars were other suns with planets, among other ideas, and who subsequently dedicated two books to Sidney.
In 1585 the couple had one daughter, Elizabeth, who later married married Roger Manners, 5th Earl of Rutland, in March 1599 and died without issue in 1614.