He was born in Rome to his father, Domenico, and his mother, Matilde Farrajoni. Parish records list his name sometimes as Meo, but Mei has been used throughout his life. His father died when Paul was three, and his mother died when he was 19; this left him with little in the way of financial resources. His initial training was under Tommaso Minardi, then a professor at the Accademia di San Luca in Rome. In 1856 he had made a strong friendship with the painter Cesare Fracassini, which would help him get many commissions. He also often collaborated with Cesare Mariani. In 1859 he contributed to decorate the rooms of Palazzo Raffaelli in Cingoli, in the province of Macerata. He also painted in Tivoli and Civitavecchia. Among his works for churches are: Chapel of the Guardian Angel in the church of Santa Maria in Aquiro; Chapel of the Crucifix in the church of Santa Lucia del Gonfalone; ceiling and side walls: only the walls remain, depicting Our Lady of Sorrows; Pala (1869) for the fourth chapel on the left in the church of Santa Maria della Pace; he completed a painting by Cesare Fracassini. The execution of S. Stefano and part of S. Lorenzo presents the properties of his church to the prefect of Rome for the Basilica of S. Lorenzo fuori le Mura both from a design and partially completed by Fracassini before his death. In 1860 he provided some drawings for the engraver Giuseppe Marcucci. He painted a series of small canvases with scenes from life in ancient Rome (also called neo-Pompeian themes). In 1865 he went to Naples, where he made contact with Domenico Morelli. After Minardi's death in 1871, he completed some of the frescoes (Art and Poetry) in the portico of the Verano cemetery (Campo Verano) in Rome. Guglielmo De Sanctis completed the other two. He is buried in this cemetery. In 1893 he was admitted as a member of the Pontifical Congregation of the Virtuosi del Pantheon. Mei died in Rome on March 21, 1900.