A full-length, highly coloured Baroque portrait slightly to left, facing to the right in a Roman costume representing Mars, the god of war. He sports a brown full-bottomed wig and gold armour of lapped plates and tassels. He also wears green three-quarter length hose and jewelled sandals, cross-gartered to lion-mask tops. His right hand rests on a stick, his left on his hip clutching a swirling, heavily embroidered red silk cloak that is fastened with a jewel on his right shoulder. He stands in a pavilion, delineated by red swags at the top and grey silk tasselled drapes to the right. In the left foreground are pieces of armour that may have belonged either to Prince Henry or to Prince Charles (his young uncle and father) about 1610. By 1660 this armour was in the Tower of London where Gascar probably sketched it. Their inclusion reinforces the imagery of the warrior, and invites a comparison between the Stuart dynasty and earlier ones.

James was Lord High Admiral for his elder brother, Charles II, from the latter's restoration to the throne in 1660 until 1673, when he was excluded from office as a Catholic under the Test Act. This position is indicated by the depiction of the fleet in the background, with the Duke's flagship 'Royal Prince', 100 guns, shown prominently at anchor. Its flags signify a royal visit to the fleet in late 1672 or early 1673, with the Admiralty flag at the fore, the Royal Standard at the main, and the Union flag at the mizzen. Normally the Duke of York would only have flown the Royal Standard at the fore. The yacht 'Anne' lies alongside, flying the Admiralty flag as an ensign. A small barge flying the red ensign is broadside on in the foreground, with another small craft in front. On the right a page, traditionally thought to be the young John Churchill, later Duke of Marlborough, or a member of the Cabal, is dressed in a similar manner to the Duke and holds a plumed helmet of Franco-Roman design.

The Duke was in personal command of the fleet at the victory over the Dutch off Lowestoft in 1665 and again at the Battle of Solebay in 1672, which this portrait may commemorate. Solebay was the last occasion in which a member of the British royal family personally commanded a fleet in action. James was an experienced and brave soldier by land and sea, and extremely interested and diligent in the administration of the Navy. He encouraged Samuel Pepys as Secretary of the Navy Board and later of the Admiralty, becoming his most important patron (other than Charles II himself) after the death of Pepys's cousin, the Earl of Sandwich, at Solebay in 1672.

He was less adept when he succeeded Charles as King of Great Britain and Ireland from 1685. His limited political judgement soon gave way to his hard-line Catholic sympathies and ruthless suppression of dissent. In 1688 he was overthrown in the very largely non-violent 'Glorious Revolution' which replaced him with his Protestant daughter, Mary, and her Dutch husband, William of Orange (as William III and Mary II). He died in exile in France after pursuing a fruitless war to regain his throne, backed by Louis XIV, though the Jacobite cause to which he gave his name smouldered on under his son, James, and grandson Charles Edward (the Old and Young Pretenders) until the latter's final defeat at Culloden in 1746.

In the tradition of the swagger portrait, this painting is an opulent statement of James's military skills. The design of the seascape visible through the drawn curtain, may be based on a lost painting by van de Velde the Younger.

The artist, Gascar, came to England in about 1672 and painted portraits with flamboyant French symbolism for the Catholic clique in the Stuart court. He worked for Charles II's mistress, Louise de Keroualle, Duchess of Portsmouth, and was also a French spy. William IV presented this portrait to Greenwich Hospital, from the Royal Collection, in 1835.