North was descended from Henry Montagu, 1st Earl of Manchester, paternal uncle of Edward Montagu, 1st Earl of Sandwich and was related to Samuel Pepys and the 3rd Earl of Bute. He at times had a slightly turbulent relationship with his father Francis North, 1st Earl of Guilford, yet they were very close. In his early years, the family was not wealthy, though their situation improved in 1735 when his father inherited property from his cousin. Frederick's mother, Lady Lucy Montagu, a daughter of George Montagu, 1st Earl of Halifax and his first wife, Ricarda Posthuma Staltonstall, died in 1734. His father remarried, but his stepmother, Elizabeth Kaye, widow of George Legge, Viscount Lewisham, eldest son of William Legge, 1st Earl of Dartmouth and his wife, Lady Anne Finch, third daughter of Heneage Finch, 1st Earl of Aylesford, died in 1745, when Frederick was thirteen. One of his stepbrothers was William Legge, 2nd Earl of Dartmouth, who remained a close friend for life.