When the English Civil War broke out in 1642 Montagu was commissioned to raise a regiment of foot, and became its youngest colonel at the age of 18.

The Royalist held Lincoln Castle was in a strategic position with its commanding elevated views overlooking the River Witham and vital routes like the Fosse Way and Ermine Street; threatening communications between Parliament and its Scottish allies; as well as allowing the King`s commander in Yorkshire, the Earl of Newcastle, to march freely down the great North Road to attack London.

On the April 20th 1644 Montagu’s regiment joined up with the “Eastern Association”, one of the four main Protestant field armies, under the command of the Earl of Manchester. They marched north to retake control of its most northern county, Lincolnshire. On May 3rd Lincoln town and castle were requested to surrender. The town and castle were under the governorship of the Royalist commander, Sir Francis Fane, who refused to do so. On May 6th Montagu’s and Russell’s regiments, were detailed to take the outworks protecting part of the town. Both regiments carried out their orders successfully, not only taking the outworks but pursuing the Royalist troops defending them and forcing them to retreat into the castle.

On the following day the upper town, around the castle, was stormed after a brief cannonade. The Royalists retreated into the castle. Scaling ladders were called up and put against the castle walls but these proved to be too short! The defenders pushed away the ladders with pike and fired carbines, pistols and muskets into the mass of Parliamentarian troops at the bottom of the wall. Eventually the attackers gained a tenuous hold on part of the wall. Soldiers then widened this hold, by climbing onto the shoulders of those at the top of the ladders, and took the castle. Those of the regiment that reached the top fought so ferociously that it demoralised the defenders and they started asking for quarter. Montagu’s regiment was praised for its valour for its part during this action.

Parliamentarian casualties were a mere eight killed and about 40 wounded. The Royalists had about 150 killed and between 650 and 800 taken prisoner. The victorious Parliamentarian soldiers rampaged through the upper city, pillaging and smashing monuments and stained glass in the cathedral.

At the end of the Civil War Montagu was elected MP for Huntingdonshire for three years. After this he fully supported the establishment of Cromwell`s Protectorate, emerging as one of its leading members.

Later, Montagu changed his loyalties to the Royalist cause, and supported the reinstatement of Charles II. The King made Montagu the first Earl of Sandwich, Knight of the Garter and Master of the Wardrobe.

Sandwich was given honourable naval duties. In early 1665, during the Second Dutch War, he had great success as a naval commander. He met his death commanding a squadron during the Battle of Solebay in May 1672 in the Third Dutch War.