Peregrine Bertie, 13th Baron Willoughby de Eresby was the son of Katherine 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, his parents had to move outside England and the boy was born at Wesel on the River Rhine.

Katherine Brandon, Duchess of Suffolk, suo jure 12th Baroness Willoughby de Eresby, was an English noblewoman living at the courts of King Henry VIII, King Edward VI and Queen Elizabeth I. She was the fourth wife of Charles Brandon, 1st Duke of Suffolk, who acted as her legal guardian during his third marriage to Henry VIII's sister Mary. Her second husband was Richard Bertie, a member of her household. Following Charles Brandon's death in 1545, it was rumoured that King Henry had considered marrying Katherine as his seventh wife, while he was still married to his sixth wife, Catherine Parr, who was Katherine's close friend.