Montgomery was born in
Surrey, in 1887, the fourth child of nine, to an
Church of Ireland
Henry Montgomery, and his wife, Maud (née Farrar).
 The Montgomerys, an '
gentry family, were the
County Donegal branch of the
Clan Montgomery. Henry Montgomery, at that time
St Mark's Church, Kennington, was the second son of
Sir Robert Montgomery, a native of
County Donegal in
 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
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
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).
It was a financial relief of some magnitude when, in 1889, Henry was made
Bishop of Tasmania, then still a
British colony and Bernard spent his formative years there.
Bishop Montgomery considered it his duty to spend as much time as possible in the rural areas of
Tasmania and was away for up to six months at a time. While he was away, his wife, still in her mid-twenties, gave her children "constant" beatings,
 then ignored them most of the time as she performed the public duties of the bishop's wife. Of Bernard's siblings, Sibyl died prematurely in Tasmania, and Harold, Donald and Una all emigrated.
 Maud Montgomery took little active interest in the education of her young children other than to have them taught by tutors brought from Britain. The loveless environment made Bernard something of a bully, as he himself recalled, "I was a dreadful little boy. I don't suppose anybody would put up with my sort of behaviour these days."
 Later in life Montgomery refused to allow his son
David to have anything to do with his grandmother, and refused to attend her funeral in 1949.
The family returned to England once for a
Lambeth Conference in 1897, and Bernard and his brother Harold were educated for a term at
The King's School, Canterbury.
 In 1901, Bishop Montgomery became secretary of the
Society for the Propagation of the Gospel, and the family returned to London. Montgomery attended
St Paul's School and then the
Royal Military College, Sandhurst, from which he was almost expelled for rowdiness and violence.
 On graduation in September 1908 he was commissioned into the 1st Battalion the
Royal Warwickshire Regiment as a
 and first saw overseas service later that year in
 He was promoted to
lieutenant in 1910,
 and in 1912 became adjutant of the 1st Battalion of his regiment at
Shorncliffe Army Camp.