Peregrine Bertie, 13th Baron Willoughby de Eresby (12 October 1555 – 25 June 1601) was the son of Catherine Willoughby, 12th Baroness Willoughby de Eresby, and Richard Bertie. Bertie was Lady Willoughby de Eresby's second husband, the first being Charles Brandon, Duke of Suffolk. Peregrine Bertie's half-brothers, Henry and Charles Brandon, died as teenagers four years before his birth. His sister Susan married the Earl of Kent and then the nephew of Bess of Hardwick. Owing to religious politics, the parents had to move outside England and the boy was born at Wesel on the River Rhine.
Born on 12 October 1555, he was baptized at the church of Saint Willibrord on 14 October. On Elizabeth I's accession to the throne in 1558, his parents returned to England and applied for a patent of naturalization for him. He formally became English on 2 August 1559. He married Mary de Vere, daughter of John de Vere, 16th Earl of Oxford, between Christmas 1577 and 12 March 1578. When his mother died in 1580, he succeeded to her barony and he took his seat in the House of Lords on 16 January 1580.
In 1582, he was commissioned to escort the Duke of Anjou from Canterbury to Antwerp. The French royal duke had arrived as a suitor of the un-married Elizabeth. In the same year he was sent to Denmark to invest Frederick II with the Order of the Garter. Lord Willoughby de Eresby arrived at Elsinore on 22 July and left on 27 September 1582. His ulterior purpose was to obtain an understanding whereby English merchant ships would not be molested while in Danish waters. In 1585, he returned to Denmark on behalf of Elizabeth in support of Henry III of Navarre and to obtain Danish help for England's efforts on behalf of the independent Netherlands.