The proto-Indo-Iranian word *mitra- could mean either "covenant, contract, oath, or treaty", or "friend". A general meaning of "alliance" adequately explains both alternatives. The second sense tends to be emphasized in Indic sources, the first sense in Iranian. The word is from a root mi- "to bind", with the "tool suffix" -tra-. A contract is thus described as a "means of binding" .

The reforms of Zoroaster retained the multitudes of pre-Zoroastrian divinities, reducing them in a complex hierarchy to "immortals" who, under the supremacy of the Creator Ahura Mazda, were now either ahuras or daevas. In this scheme, Mithra is a member of the ahuric triad, protectors of asha, the order of the universe. Mithra is additionally the protector of truth and justice and the source of cosmic light. In Middle Persian Mithra came to be known as Meher.

Mithra is not present in the Gathas of Zoroaster (Zarathustra) but appears in the younger Yashts of the Avesta (Campbell p 257). There, Mithra comes to the fore among the created beings. "I created him" Ahura Mazda declares to Zoroaster, "to be as worthy of sacrifice and as worthy of prayer as myself" (Campbell, loc. cit.). In the Yashts, Mithra gains the title of "Judge of Souls" and is assigned the domain of human welfare (which he shares with the Creator). Mithra occupies an intermediate position in the Zoroastrian hierarchy as the greatest of the yazata, created by Ahura Mazda (Ormuzd in later Persian) to aid in the destruction of evil and the administration of the world. He is then the divine representative of the Creator on earth, and is directed to protect the righteous from the demonic forces of Angra Mainyu (Ahriman in later Persian).

As the protector of truth and the enemy of error, Mithra occupied an intermediate position in the Zoroastrian pantheon as the greatest of the yazatas, the beings created by Ahuramazda to aid in the destruction of evil and the administration of the world. He was thus a divinity of the realms of air and light, and, by transfer to the moral realm, the manifestation of truth and loyalty. As the enemy of darkness and evil spirits, he protected souls, accompanying them to paradise, and was thus a redeemer. Because light is accompanied by heat, he was the promoter of vegetation and increase; he rewarded the good with prosperity and annihilated the bad.

While in older Zoroastrianism Mithra is seen as a creation of Ahura Mazda, in later Persian culture, Mithra evolved to be an incarnation of Ahura Mazda , and in his role as 'Judge of Souls' as the rewarder of good and annihilator of the bad. Mithra was seen as omniscient, undeceivable, infallible, eternally watchful, and never-resting.

Similarly, while in the Sirozeh, Mithra is also referred to as Dae-pa-Meher, or Creator of Meher, this separation between 'Meher' and the 'Creator of Meher' dissolves in later texts and the distinguishing characteristics of Mithra and Meher blend. Mithra, reincorporated as "Meher", thus also becomes the representative of truth and justice, and, by transfer to the physical realm, the divinity of air and light. As the enemy of darkness and evil spirits, he protected souls, a psychopomp accompanying them to paradise. As heat accompanying light, Mithra became associated with growth and resultant prosperity.

Mithra worship spread first with the empire of the Persians throughout Asia Minor, then throughout the empire of Alexander and his successors.

The Parthian princes of Armenia were hereditary priests of Mithra, and an entire district of this land was dedicated to Anahita. Many temples were erected to Mithra in Armenia, which remained one of the last strongholds of the Mazdaist cult of Mithra until it became the first officially Christian kingdom.