This is a fragment from the left side of the central panel of a large altarpiece, known as the Santa Trinità Altarpiece. It was commissioned in September 1455 by the Company of Priests of the Trinity for their church in Pistoia, Italy.

The choice of saints in the altarpiece was specifically requested by the priest in charge. Mamas, here a young shepherd boy, stands on the left next to James the Greater, who carries a book and staff. The lion, peering out from behind the figures, is a reference to Saint Mamas who was thrown to the lions during his time in prison.  The remainder of the altarpiece represents the Holy Trinity along with Saints Zeno and Jerome, with a predella below.

When Pesellino died in July 1457, the altarpiece was finished by the artist Fra Filippo Lippi (born about 1406; died 1469). It was later divided into several parts, probably in the eighteenth century, and the altarpiece has now been largely reassembled at The National Gallery, London.