He was the eldest son of John Montagu, 3rd Earl of Salisbury (died 1400), who was killed while plotting against King Henry IV in 1400, and his lands forfeited.

In July 1415, he was one of the seven peers who tried Richard, Earl of Cambridge on charges of conspiring against King Henry V. Montagu then joined King Henry V in France, where he fought at the siege of Harfleur and at the Battle of Agincourt.

Lady Margaret Montacute Lady Elizabeth Montacute Sir Alan Buxhull II, Half Brother[3] He had an illegitimate son John (or James) Montagu/Montacute, to whom he bequeathed his manor of Luddesdown in Kent.

On 27 October 1428, he was wounded during the siege of Orléans, when the tower he was inside was hit by a cannonball.

There are conflicting reports on the manner in which this wounded him; Enguerrand de Monstrelet states a piece of stone from the window 'carried away part of his face'.