Although Darius consolidated and added to the conquests of his predecessors, it was as an administrator that he made his greatest contribution to Persian history. He completed the organization of the empire into satrapies, initiated by Cyrus the Great, and fixed the annual tribute due from each province. During his reign, ambitious and far-sighted projects were undertaken to promote imperial trade and commerce. Coinage, weights, and measures were standardized and land and sea routes developed. An expedition led by Scylax of Caryanda sailed down the Indus River and explored the sea route from its mouth to Egypt, and a canal from the Nile River to the Red Sea, probably begun by the chief of the Egyptian delta lords, Necho I (7th century BC), was repaired and completed.

While measures were thus taken to unite the diverse peoples of the empire by a uniform administration, Darius followed the example of Cyrus in respecting native religious institutions. In Egypt he assumed an Egyptian titulary and gave active support to the cult. He built a temple to the god Amon in the Kharga oasis, endowed the temple at Edfu, and carried out restoration work in other sanctuaries. He empowered the Egyptians to reestablish the medical school of the temple of Sais, and he ordered his satrap to codify the Egyptian laws in consultation with the native priests. In the Egyptian traditions he was considered as one of the great lawgivers and benefactors of the country. In 519 BC he authorized the Jews to rebuild the Temple at Jerusalem, in accordance with the earlier decree of Cyrus. In the opinion of some authorities, the religious beliefs of Darius himself, as reflected in his inscriptions, show the influence of the teachings of Zoroaster, and the introduction of Zoroastrianism as the state religion of Persia is probably to be attributed to him.

Darius was the greatest royal architect of his dynasty, and during his reign Persian architecture assumed a style that remained unchanged until the end of the empire. In 521 BC he made Susa his administrative capital, where he restored the fortifications and built an audience hall (apadana) and a residential palace. The foundation inscriptions of his palace describe how he brought materials and craftsmen for the work from all quarters of the empire. At Persepolis, in his native country of Fars (Persis), he founded a new royal residence to replace the earlier capital at Pasargadae. The fortifications, apadana, council hall, treasury, and a residential palace are to be attributed to him, although not completed in his lifetime. He also built at Ecbana and Babylon.