Balthasar Charles was born on the 17 October 1629, the only son of King Philip IV and Isabella of Bourbon. He was heir apparent to all the kingdoms, states and dominions of the Spanish monarchy, with titles including Prince of Asturias, Prince of Girona, Duke of Montblanc, Count of Cervera, Lord of Balaguer and Prince of Viana. Don Balthasar Charles died of smallpox on the 9 October 1646. This portrait of the Infante is attributed to Juan Bautista Martínez del Mazo and is one of two known versions of the composition. Another version, attributed to the studio of Mazo, is presently on long term loan to the Rijksmuseum, Amsterdam from the Mauritshuis, The Hague (Inv. No. 298). A third version was offered for sale in 2015.

The portrait, showing the Balthasar Carlos aged about eleven, was first recorded in Royal Collection in the 1639 inventory of Charles I as 'the present Prince of Spain, given to the Queen'; Queen Henrietta Maria was the Infante's the maternal aunt. It was almost certainly sent from the Spanish court, along with portraits of King Philip IV and Queen Isabella of Bourbon (RCINs 402914 and 402915). While the portrait of Balthasar Carlos and that of Philip IV appear to have been painted by del Mazo, the portrait of Isabella of Bourbon is in a different hand. All three paintings have been extended with extra strips of canvas. While the texture of the additional canvas is different to that of the central section, in all three portraits the paint surface and technique is continuous over the joins, suggesting that the additions were completed at a very early date in the history of the paintings, probably before they left the artist's studio.

The children of the Spanish Habsurgs and the higher echelons of Spain's nobility were instructed to behave in both a formal and upright, Catholic manner from a very young age. This is reflected in the court portraiture of the younger members of the Habsburg House. Rather than being portrayed as child-like, they appear conscious of their status and exemplify princely virtues. In this portrait Balthasar Carlos is depicted wearing ornate armour and riding boots and holding a commander's baton, alluding to his aptitude for the military duties he will one day assume as king.

Del Mazo was the son-in-law of Velázquez. He probably entered the studio of Velazquez in 1630 and married his Master's daughter in 1633. He assisted Velázquez in the production of court portraits, working in a manner close to that of his father-in-law. He served as painter to Prince Balthasar Carlos and succeeded Velazquez as court painter (Pintor de Cámara) to Philip IV of Spain and on the death of the old king in 1665, fulfilled the role under Carlos II until his own death in 1667. Although his life is well documented, Mazo's style so closely imitated that of his father-in-law that very few surviving works can be securely attributed to him.