In about 1292 he married Elizabeth de Montfort (died August 1354), daughter of Peter de Montfort (died before 4 March 1287) (son of Peter de Montfort of Beaudesert Castle by Alice Audley) by his wife Maud de la Mare, daughter and heiress of Sir Henry de la Mare (died 1257), of Ashtead, Surrey, Hinton (in Hurst), Diddenham (in Shinfield), Farley Hill, and Sheepbridge (in Swallowfield), Berkshire, Justice and Seneschal to William Longespée, 2nd Earl of Salisbury, by his wife Joan Neville (daughter of Sir John Neville of Essex by his wife Hawise de Courtenay, daughter of Robert de Courtenay (died 1242), feudal baron of Okehampton in Devon]).

Elizabeth de Montfort survived her husband and remarried to Sir Thomas Furnivall (d. before 18 April 1332) of Sheffield, who was pardoned and fined £200 on 8 June 1322 for marrying her, a widow of a tenant-in-chief, without royal licence.
Furnivall's son, Thomas Furnivall (d. October 1339), had married Joan de Verdun (d. 2 October 1334), widow of Elizabeth's eldest son, John Montagu.
Elizabeth died in August 1354, and was buried in the Priory of St Frideswide, Oxford (now Christ Church Cathedral, Oxford), where her tomb still exists in the Latin chapel.

By his wife Montagu had four sons and seven daughters:
In about 1292 he married Elizabeth de Montfort (died August 1354), daughter of Peter de Montfort (died before 4 March 1287) (son of Peter de Montfort of Beaudesert Castle by Alice Audley) by his wife Maud de la Mare, daughter and heiress of Sir Henry de la Mare (died 1257), of Ashtead, Surrey, Hinton (in Hurst), Diddenham (in Shinfield), Farley Hill, and Sheepbridge (in Swallowfield), Berkshire, Justice and Seneschal to William Longespée, 2nd Earl of Salisbury, by his wife Joan Neville (daughter of Sir John Neville of Essex by his wife Hawise de Courtenay, daughter of Robert de Courtenay (died 1242), feudal baron of Okehampton in Devon]).
Elizabeth de Montfort survived her husband and remarried to Sir Thomas Furnivall (d. before 18 April 1332) of Sheffield, who was pardoned and fined £200 on 8 June 1322 for marrying her, a widow of a tenant-in-chief, without royal licence. Furnivall's son, Thomas Furnivall (d. October 1339), had married Joan de Verdun (d. 2 October 1334), widow of Elizabeth's eldest son, John Montagu. Elizabeth died in August 1354, and was buried in the Priory of St Frideswide, Oxford (now Christ Church Cathedral, Oxford), where her tomb still exists in the Latin chapel.
By his wife Montagu had four sons and seven daughters:
William Montagu, 2nd Baron Montagu