Plutarch tells that Numa was the youngest of Pomponius' four sons, born on the day of Rome's founding (traditionally, 21 April 753 BC). 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 13 years of marriage, Tatia died, precipitating Numa's retirement to the country. According to Livy, Numa resided at Cures immediately before being elected king.

Livy and Plutarch refer to and discredit the story that Numa was instructed in philosophy by Pythagoras, as incompatible with dates.

Plutarch reports that some authors credited him with only a single daughter, Pompilia, others also gave him five sons, Pompo (or 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 favour with those families. Pompilia, whose mother is variously identified as Numa's first wife Tatia or his second wife Lucretia, supposedly married a certain Marcius and by him gave birth to the future king, Ancus Marcius.

After the death of Romulus, there was an interregnum of one year in which the royal power was exercised by Senate members in rotation for a single day in a row. In 715 BC after much bickering and as the result of a compromise between the Roman (Romulus - originating) and Sabine (Tatius - originating) factions, Numa, himself a Sabine, was elected by the Senate of Rome to be the next king.

According to Plutarch, Numa was a cunning and calculating person; he at first refused the offer, however his father and Sabine kinsmen, and the Roman envoys (two senators) banded together to persuade him to accept. Plutarch and Livy recount how Numa, after being summoned by the Senate from Cures, and proferred the tokens of power within a popular surge of enthusiasm requested that prior to his acceptance an augur to divine the opinion of the gods on the prospect of his kingship should be taken. Jupiter was consulted and the omens were favourable. Thus placated by the Roman and Sabine people on the one hand and anointed by the heavens, he took up his position as King of Rome.

According to Plutarch, Numa's very first act was to disband the personal guard of 300 so-called "Celeres" (the "Quick - stepped") that Romulus permanently entertained about himself. Whether self protectory (future Roman history will demonstrate that such guards were a double - sided weapon), or a sign of humility, or a signal of peace and moderation for all to understand is not certain.

Beyond seeing to having his access to power endorsed by Jupiter, Numa is credited with sustaining a permanent and direct relationship with a variety of deities, mostly of lower order, in the person of the nymph Egeria and also some of the Muses, but also when required by circumstances, with the greater powers, and especially Jupiter himself, most of the time guided by Egeria in order to avoid mistakes, misinterpretations or traps.

Numa was later celebrated for his natural daily wisdom and piety; legend says the nymph Egeria taught him to be a wise legislator. According to Livy, Numa pretended that he held nightly consultations with the goddess Egeria on the proper manner of instituting sacred rites for the city. Plutarch suggests that he played on superstition to give himself some awing allure, in order to better bridle the ferocious instincts of early Romans and bend them to more gentle behaviours, such as honoring the gods, abiding by laws, behaving humanely to foes and living proper, respectable lives.

Numa allegedly authored several "sacred books" in which he had written down the teachings and recommendations of the deities, mostly Egeria and the other Muses, he interacted with. Plutarch, citing Valerius Antias, and Livy mention that he was buried along with these "sacred books" (they don't agree about the numbers of them) at his request, on the ground that what they ordered was entirely known to the priests and that he would rather the rules and rituals they prescribed be kept by a living memory and practice than preserved as relics doomed for forgetfulness and disuse. Of those about half were thought to be dedicated to the ministry of the several orders of priests (flamines, pontifices, Salii, Fetiales...) he had set up or adapted, and as to the rest, was not recorded precisely. According to Plutarch, these books were recovered some four hundred years later at the occasion of some natural accident that unveiled the tomb, were examined by the Senate, and deemed to be inappropriate for disclosure to the people and hence burned; Dionysius of Halicarnassus hints that they were actually kept as a very close secret by the Pontifices.

Numa is reputed to have constrained two minor gods, Picus and Faunus into delivering some prophecies of things to come.

Numa, supported and prepared by Egeria, is reported to have held a battle of wits with Jupiter himself, in an apparition whereby Numa sought to gain a protective ritual against lightning strokes and thunder.

At a time of a pestilential disease that was playing havoc in the population, a prodigy happened: a shield which had undecypherable letters written on it, came to fall from the sky on the Palatine Hill. This was brought to Numa, and he declared that Egeria had enlightened him that this was a token of safeguard from Jupiter, for which he organized due measures of recognition, thus bringing the plague at an immediate end. This shield became a sacred relic of the Romans.

One of Numa's first acts was the construction of a temple of Janus as an indicator of peace and war. The temple was constructed at the foot of the Argiletum, a road in the city. After securing peace with Rome's neighbours, the doors of the temples were shut and remained so for all the duration of Numa's reign, a unique case in Roman history.

Another, surprising, creation attributed to Numa was the cult of Terminus, a god for boundaries; through this rite, which involved sacrifices at private properties boundaries and landmarks, Numa reportedly sought to instill in Romans the respect of lawful property and non-violent relationships with neighbours; the cult of Terminus, preached Numa, involved absence of violence and murder, and that god was testimony to justice and keeper of peace, in a somehow comparable, more moral rather than legal mode, way to one of the roles of Vegoia in the neighbouring Etruscans' religious system; setting official boundaries to the territory of Rome, which Romulus had never wanted, was also, presumably with the same concern of preserving peace, also decided by Numa.

Recognizing the paramount importance of the sacred shield descended from the skies, King Numa had eleven matching shields made, so perfect that no one, even Numa, could distinguish the original any longer. These shields were the ancilia, the sacred shields of Jupiter, which were carried each year in a procession by the Salii priests. Numa also established the office and duties of Pontifex Maximus and instituted (Plutarch's version) the flamen of Quirinus, in honour of Romulus, in addition to those of Jupiter and Mars that already existed . Numa also brought the Vestal Virgins to Rome from Alba Longa.; Plutarch adds that they were then at the number of two, were later augmented to four by Servius Tullius and stayed so through the ages.

By tradition, Numa promulgated a calendar reform that adjusted the solar and lunar years, introducing the months of January and February.

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:

"So, distinguishing the whole people by the several arts and trades, he formed the companies of musicians, goldsmiths, carpenters, dyers, shoemakers, skinners, braziers, and potters; and all other handicraftsmen he composed and reduced into a single company, appointing every one their proper courts, councils, and religious observances." (Plutarch)

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.