A three-quarter-length portrait seated and facing forward to meet the gaze of the viewer. He wears a brown silk gown, a red flower-embroidered waistcoat and a fair, full-bottomed wig. His left hand rests on a paper on which is written, 'Memoires of my owne life 1708'. There is also an inkwell and a pen on the table. A sword, cane and a pair of pistols hang on the wall behind him on the left, while on the right is a lodestone attached to a little anchor, and a quadrant.

Knox was the son of Robert Knox, a sea-captain in the service of the English East India Company. In 1659 Robert senior was returning home from his post with the Company at Fort St George (Madras), accompanied by his son, when a storm forced their ship, the 'Ann', into Cottier Bay, Ceylon. They were taken prisoner with the rest of the crew by the Sultan of Ceylon and Knox senior died of malaria in captivity. After twenty years, during which he survived by knitting caps, hawking goods and lending rice and corn, young Robert escaped with one companion in 1679 and reached Arippu, a Dutch settlement on the north-west coast of the island. The following year he joined the East India Company and in turn served as a sea-captain until he was dismissed the service in 1694. In revenge he took command of an interloping trading vessel, the 'Mary', and he died a wealthy bachelor in London. In 1681 Knox also wrote an account of his experiences, 'An Historical Relation of the Island Ceylon, in the East-Indies', accompanied by engravings showing the inhabitants, their customs and agricultural techniques.

In 1689, it was from him that Robert Hooke, Secretary of the Royal Society, learnt of the existence of 'ganga', 'a strange intoxicating herb like hemp', of which Knox gave him seeds and on which Hooke addressed the Society that December. This was the first English report of ' Indian hemp' or cannabis. The painting is inscribed 'R: Knox' on the quadrant , 'aetatis 66' on the left, and is signed 'P.Trampo: pinx' on the chair. An engraved portrait of Knox by 'R.White ad vivum', dated 1695, is also known.