Montgomery was born in Kennington, Surrey, in 1887, the fourth child of nine, to an Ulster-Scots Church of Ireland minister, The Reverend Henry Montgomery, and his wife, Maud (née Farrar). The Montgomerys, an 'Ascendancy' gentry family, were the County Donegal branch of the Clan Montgomery. Henry Montgomery, at that time Vicar of St Mark's Church, Kennington, was the second son of Sir Robert Montgomery, a native of Inishowen in County Donegal in Ulster, the noted colonial administrator in British India, who died a month after his grandson's birth. He was probably a descendant of Colonel Alexander Montgomery (1686–1729). Bernard's mother, Maud, was the daughter of The V. Rev. Frederic william Canon Farrar, the famous preacher, and was eighteen years younger than her husband. After the death of Sir Robert Montgomery, Henry inherited the Montgomery ancestral estate of New Park in Moville in Inishowen in Ulster. There was still £13,000 to pay on a mortgage, a large debt in the 1880s, and Henry was at the time still only an Anglican vicar. Despite selling off all the farms that were at Ballynally, "there was barely enough to keep up New Park and pay for the blasted summer holiday" (i.e., at New Park).