Harcourt was the eldest son of Robert Harcourt and Frances, daughter of Geoffrey Vere, third son of John de Vere, 15th Earl of Oxford. Succeeding to a somewhat embarrassed estate, he sought a military career abroad. At the age of sixteen he served under his uncle, Sir Horace Vere, against the Spanish forces in the Low Countries, and was knighted at Whitehall Palace on 26 June 1627. The greater part of his life was spent in Holland in the service of the Prince of Orange. He was also in favour with Elizabeth of Bohemia, who commended him to Archbishop Laud, when business of a domestic nature (connected probably with the recovery of Stanton Harcourt) obliged him to repair to England in 1636. Though holding a commission as sergeant-major from the Prince of Orange, he took an active part in the Bishops' Wars against Scotland in 1639–40, as commander of a regiment of foot. A diary kept by him during this campaign still exists.