A three-quarter-length portrait to right in a leather coat and breastplate, with a red and gold sash is round his waist. He wears the ribbon of the Garter over his right shoulder to which is attached the Lesser George. His hair is worn long and his right hand rests on a cannon. He stands against a wall with purple drapery on the left and there is a ship on fire on the right.

Montagu's friendship with Cromwell assured his advancement and in 1656 he was appointed joint General-at-Sea with Blake and later in sole command of the fleet in the Downs. After the death of Cromwell and the fall of his son Richard, Montagu transferred his allegiance to Charles II. He commanded the fleet which fetched Charles from Holland in 1660, for which he was created Earl of Sandwich. In 1661 he brought Catherine of Braganza from Portugal. In the Second Dutch War, Sandwich was Admiral of the Blue at the Battle of Lowestoft but his part in the failure to take the Dutch East India fleet at Bergen forced him to stand down. On the renewal of war with the Netherlands in 1672, he again commanded the blue squadron, which led the fleet into action at Solebay on 28 May. A fireship set alight his flagship, the 'Royal James', 100 guns, and he was drowned when the boat transferring him to another ship sank. His body was recovered some time later, identified by the Garter star on his breast. The portrait is inscribed, 'The Earle of Sandwich', and was commissioned by Charles II's brother James, Duke of York after the Battle of Lowestoft. This was the first major action of the Second Dutch War, in which James commanded the fleet.

It is one of the 'flagmen' portraits of which others were seen by Pepys in Lely's studio on 18 April 1666. His diary entry notes this and two others (of Smith and Penn) as not yet begun. Sandwich had then already left to be Ambassador at Madrid, so the portrait could not have been finished until 1668 unless Lely worked from old sketches or drawings. If the burning ship is intended to be the 'Royal James', then the portrait must have been finished posthumously.

Lely, a Dutchman who arrived in England in 1641 after the death of Van Dyck, soon became his successor as leading portraitist of the day. He worked for Charles I, continued to flourish under the Commonwealth and Protectorate, and after the Restoration of 1660 was appointed Principal Painter to Charles II. The full 'flagmen' set consists of thirteen individual portraits, of which George IV presented eleven plus a copy of that of Admiral Sir John Lawson (BHC2833) to Greenwich Hospital in 1824. The originals of Lawson and of Prince Rupert were retained in the Royal Collection, although William IV presented an extended full-length copy of the latter (BHC2990) to the Hospital in 1835.