In 1368, a member of the Arte dei Medici e Speziali in Florence is identified as Niccolò dipintore. Gerini is recorded as collaborating with Jacopo di Cione on frescoes for the Guild hall of the Judges and Notaries in Florence in 1366. He is the Niccolaio dipintore who worked with Jacopo di Cione on the altarpiece of the Coronation of the Virgin commissioned by Albizzi family for St Pier Maggiore in Florence in 1370. He was paid 12 golden florins "per disegnare la tavola dell’altare" in November of the same year. He designed the altarpiece and the elaborate throne canopy while Jacopo di Cione was depicting side saints.
He collaborated with Jacopo di Cione on a second Coronation of the Virgin (Accademia, Florence) in 1372. It was commissioned by the mint of Florence Zecca Vecchia on the same year. In 1383 Gerini again worked with Cione on a fresco of the Annunciation in the Palazzo dei Priori, Volterra. This fresco clearly shows the work of two very different artists: Niccolò di Pietro Gerini (design and very fine painting) and Jacopo di Cione (broadly painted saints and side decoration). In 1386 Niccolò frescoed the façade of the Bigallo, Florence. He also frescoed Sant'Ambrogio church in Florence. Gerini performed the Crocefissione of St Felicita church in Florence.
His hand is clearly on sacrestia of the basilica of Saint Croce to Florence with Scenes of the life of Christ. Between 1391 and 1392 he worked in Prato where he frescoed Palazzo Datini, church of Saint Francisco with Lorenzo di Niccolo and Agnolo Gaddi.
He also frescoed capitals of the church of Saint Francisco, Pisa.
He represents giottesque school, in the tradition of the Andrea di Orcagna (1320-1368) and of Taddeo Gaddi. His father Pietro Geri is registered as a member of Lucas Guild in 1339. Niccolo worked mainly in Florence, although he also carried out commissions in Rome (Vatican), Pisa and Prato. Very typically for Gothic depiction Gerini's figures have large chins, sloping foreheads, and sharp noses whilst their bodies are squat and frontally displaced.
Another important artist Lorenzo di Niccolò di Martino was trained in Niccolò di Pietro Gerini's workshop and later collaborated with the master but was not his son as sometimes erroneously stated. Gerini though had a son Bindo di Niccolo di Pietro Gerini, born in 1363, who is registered as member of Lucas Guild since 1408.
Niccolo di Pietro Gerini works can be found in major art galleries of Rome, Vatican, Florence, London, Milan, New York, Los Angeles, Amsterdam, Berlin, Paris, St Petersburg, Boston, Cambridge, Budapest, Birmingham, Pegalo, Prato, Pisa, Altenburg, Avignon, Denver and several other museums.