The sitter in this full-length portrait has been identified as Luis Mendes de Vasconcellos on the basis of the coat of arms in the upper right corner, which belongs to the Vasconcellos family. Luis Mendes de Vasconcellos was a Portuguese nobleman; a skilled soldier with long service in India and the author of a treatise on the art of war. He was Governor of Portuguese Colonial Angola between 1617 and1621. After returning to Europe he was appointed the 55th Grand Master of the Order of Saint John of Jerusalem (The Sovereign Military Hospitaller Order of Saint John of Jerusalem of Rhodes and of Malta) in 1622, although he died within six months of taking up the position. The white cross of the Order of Saint John has been integrated into the upper part of his coat of arms.

Several known portraits of Mendes de Vasconcelos survive. An engraved portrait, which appeared in the Histoire des Chevaliers Hospitaliers de S. Jean de Jerusalem (1726), appears to have been based on an untraced painted portrait, possibly made at the time of the sitter's appointment as Grand Master of the Order of Saint John of Jerusalem. In the Royal Collection portrait, the sitter appears younger and is shown as a soldier, rather than in the costume of the Order of Saint John of Jerusalem, as in other known portraits. The parcel-gilt armour with a peascod belly suggests a date between 1570 and 1610. The apparent age of the sitter suggests a date of c. 1580. Mendes de Vasconcelos wears fashionable white silk paned trunk-hose padded into a distinctive triangular shape and holds a baton of command in his right hand, while his left rests on the hilt of his sword. His helmet sits on the base of stone column and is decorated with multiple ostrich plumes of different colours.

The painting was previously attributed to Philip III's court painter, Juan Pantoja de la Cruz; however, this attribution has subsequently been called into question.

Technical examination of the painting has shown additional strips of canvas have been added to the picture on both sides; 20cm left and 19.5cm on the right. The canvas of these additions is finer than that of the main central section and the painting technique is dissimilar. Reasons for this variation in construction are unclear but the dimensions of the picture given in the 1666 inventory of Charles II (7ft 3in x 4ft 2in / 221 x 127cm) indicate that the additions were made after this date. Inside the additional strip of canvas on the right, another strip 22cm wide has been added. This appears to be part of the original composition as the canvas is the same texture as that of used for the main central section and the join appears to be sewed. This seemingly early addition includes the coat of arms and much of the column and plumed helmet.