A three-quarter-length portrait to left in a black coat. His sword hangs from a heavy gold baldric and he holds a baton in his right hand. With his left hand he gestures towards the stern of a ship at sea and he stands in front of a stone wall on the right.

In the First Dutch War (1652-54), Penn commanded squadrons at the Battles of the Kentish Knock, Portland, the Gabbard and Scheveningen, and he received a gold medal and chain from Parliament for his service (both also in the NMM collection). Despite the fact that he took Jamaica for Parliament in 1655, he fell out of favour in his return and was one of those who backed the Restoration of Charles II, when he was dispatched with Sandwich to fetch the king from Holland in the 'Naseby', 86 guns, and was made a Commissioner of the Admiralty.

As captain of the fleet at the Battle of Lowestoft in 1665, he was in the flagship of James, Duke of York, who was in overall command. His experience and advice to the Duke did much to ensure the victory. Penn's son, a Quaker dissenter, was in 1682 the founder of the English colony (now American state) of Pennsylvania - the 'Penn' prefix being added by specific command of Charles II in honour of his father.

The portrait is inscribed 'Sir Will:Penn', and is one of the 'flagmen' portraits commissioned by James, after the battle, which was the first major action of the Second Dutch War. Others of the series were seen by Pepys on 18 April 1666 in Lely's studio, when he noted this one and those of Smith and Sandwich as yet to be begun. 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.