He was the son of Robert de Vere, 19th Earl of Oxford and his wife Beatrix van Hemmend. He was educated at Friesland in the Netherlands after his father was mortally wounded at the Capture of Maastricht in 1632, when de Vere was only six years old; years later he joined the English Regiment of Foot serving on the continent with the Dutch. He remained in Holland during the period of the English Civil War, but returned to England in 1651 an ardent royalist. He was involved in a succession of plots, for which he was imprisoned in the Tower of London for allegedly plotting against Oliver Cromwell and interned without trial. On release he joined Sir George Booth's rising in 1659 against Richard Cromwell's regime.