Numa was appointed to the position of king by the people. He built the Regia that was first meant to be a residence of the king, but Numa was so involved in religious matters that this building became the official headquarters of the Pontifex Maximus, the chief priest of Rome. The Regia was a public and religious archive, containing information on how to perform sacred rights, the state calendar of holy days, the record of events of each year, and laws relating to marriage, death, and wills. The Roman people acknowledged King Numa with having set up the ancestral forms of worship, which were distinctive of their race and their civilization. Numa believed that fire was the primary principle of the universe, that blood sacrifice was odious to the deities, and that peace was the highest civic goal. He abolished all blood sacrifices, ordaining that only grains, fruits, and other products of agriculture should be offered on the altars.

Numa is also credited with diverting the Etruscans from thoughts of war to a peaceful existence.

Numa built the temple of Janus, whose doors were open during war to symbolize soldiers marching out. Janus was a god of doorways, including the gates at the walls of Rome. The greatest of all his works was the preservation of peace and the security of his realm throughout the whole of his reign.

Under his administration, there was peace throughout Italy for forty-three years.