Before 1296, he was a squire in the service of Gilbert de Clare, Earl of Gloucester and Hertford and his wife Joan of Acre, the daughter of Edward I.
Thomas Walsingham relates that, while pleading for her husband, Joan told her father "No one sees anything wrong if a great earl marries a poor and lowly woman.
Monthermer then paid homage to Edward at Eltham Palace and was formally recognised as jure uxoris Earl of Gloucester and Hertford.
Of fine gold with three red chevrons, He had there only a banner;[nb 1] Yet he made no bad appearance, When he was attired in his own arms, Which were yellow with a green eagle.
[1] In February 1301, Monthermer was summoned to a parliament at Lincoln, specially convened for the purpose of composing the Barons' Letter of 1301, which rejected Pope Boniface VIII's claim to the feudal overlordship of Scotland.
After the Battle of Bannockburn in 1314, at which Monthermer fought and was captured, Robert, now the victorious King of Scots, discharged the debt by releasing Ralph without ransom, but not before first entertaining him at table.
His second wife was Isabel le Despencer, the widow of Lord Hastings and a daughter of the Earl of Winchester, whom he married around 1313, also in secret; for this further transgression, he was not pardoned until 1319.