Plutarch tells that Numa was the youngest of Pomponius's four sons, born on the day of Rome's founding. He lived a severe life of discipline and banished all luxury from his home. Titus Tatius, King of the Sabines and a colleague of Romulus, married his only daughter, Tatia, to Numa. After thirteen years of marriage, Tatia died, precipitating Numa's retirement to the country. Plutarch reports that some authors credited him with only a single daughter, Pompilia, others also gave him five sons, Pomponius, Pinus, Calpus, Mamercus and Numa, from whom the noble families of the Pomponii, Pinarii, Calpurnii, Aemilii and Pompilii respectively traced their descent. Other writers believed that this was merely a flattery invented to curry favor. Pompilia, whose mother is variously named as Numa's first wife Tatia and his second wife Lucretia, supposedly married Marcius II and had the future King Ancus Marcius.
Numa was around forty when he was offered the kingship. He was residing "at a famous city of the Sabines called Cures, whence the Romans and Sabines gave themselves the joint name of Quirites" (Plutarch). Though he first refused, his father and Marcius I (Marcius II's father) persuaded him to accept.
Numa was later celebrated for his natural wisdom and piety; legend says the nymph Egeria taught him to be a wise legislator. Wishing to show his favour, the god Jupiter caused a shield to fall from the sky on the Palatine Hill, which had letters of prophecy written on it, and in which the fate of Rome as a city was tied up. Recognizing the importance of this sacred shield, King Numa had eleven matching shields made. These shields were the ancilia, the sacred shields of Jupiter, which were carried each year in a procession by the Salii priests.
By tradition, Numa promulgated a calendar reform that adjusted the solar and lunar years, and he established the original constitution of the priests, called Pontifices. In other Roman institutions established by Numa, Plutarch thought he detected a Laconian influence, attributing the connection to the Sabine culture of Numa, for "Numa was descended of the Sabines, who declare themselves to be a colony of the Lacedaemonians."
Numa was credited with dividing the immediate territory of Rome into pagi and establishing the traditional occupational guilds of Rome:
Numa also instituted the Vestal Virgins.
Plutarch, in like manner, tells of the early religion of the Romans, that it was imageless and spiritual. He says Numa “forbade the Romans to represent the deity in the form either of man or of beast. Nor was there among them formerly any image or statue of the Divine Being; during the first one hundred and seventy years they built temples, indeed, and other sacred domes, but placed in them no figure of any kind; persuaded that it is impious to represent things Divine by what is perishable, and that we can have no conception of God but by the understanding".
Numa Pompilius died in 673 BC of old age. He was succeeded by Tullus Hostilius.
His history is considered legend because of a number of inconsistencies in the data historically recorded about him. The most famous was that he was a friend of Pythagoras, who is traditionally thought to have died around 500 B.C.There are many related to Pompilius, including Julia Pompilius.