From the fresco on the vault above the high altar. An inscription above his desk identifies the saint.

This painting forms part of a diptych, accompanying a Madonna of Humility.
Lorenzo has portrayed the saint as a monk in a study, engaged in translation, rather than as a cardinal.
Saint Jerome's lecturn has two shelves, one to hold the book being translated, the other the translation.

The donor nun, in her Dominican habit, has had herself painted herself on the study side, rather than the penitential side. St Jerome is a good deal less unkempt (beard trimmed and so on) in his study - even the lion looks less mangy.
The colours are as usual - bright red hat, yellow lion, etc. Andrea di Giusto may have been an assistant of Masaccio.

An inventory of 1492 records this painting hanging in the scrittoio of Lorenzo il Magnifico. It was painted for Cardinal Albergati (died 1443), who is identifed in the inscription on the letter on the table.
The books and other objects relate both to the saint's intellectual pursuits and to religious symbolism. The jar labelled tyriaca (an antidote for snakebite) surmounted by a pomegranate (a symbol of the resurrection) refers to Christ as the saviour of the world.
The influence of this work can be seen in Ghirlandaio's fresco in the Ognissanti.

Saint Jerome is surrounded by books, an inkwell and a crucifix.
His lion is seen in the shadows on the right, and there are also a cat, a peacock (a symbol of immortality) and a partridge (an allusion to the truth of Christ).

This work has a pendant fresco of St Augustine by Botticelli on the other side of the church.

Saint Jerome translated the Bible into Latin from the original Greek and Hebrew.
The pink and blue of Jerome's robes and the blue of his cardinal's hat are unusual in representations of the saint. Until the 1460's it was customary for cardinals to wear a violet or blue cape unless granted the privilege of wearing red when acting on papal business.