In the Heliopolitan creation myth, Atum was considered to be the first god, having created himself, sitting on a mound (benben) (or identified with the mound itself), from the primordial waters (Nu). Early myths state that Atum created the god Shu and goddess Tefnut by spitting them out of his mouth. Other myths state Atum created by masturbation, with the hand he used in this act representing the female principle inherent within him. Yet other interpretations state that he has made union with his shadow.

In the Old Kingdom, the Egyptians believed that Atum lifted the dead king's soul from his pyramid to the starry heavens. He was also a solar deity, associated with the primary sun god Ra. Atum was linked specifically with the evening sun, while Ra or the closely linked god Khepri were connected with the sun at morning and midday.

In the Book of the Dead, which was still current in the Graeco-Roman period, the sun god Atum is said to have ascended from chaos-waters with the appearance of a snake, the animal renewing itself every morning.

Atum is the god of pre-existence and post-existence. In the binary solar cycle, the serpentine Atum is contrasted with the scarab-headed god Khepri—the young sun god, whose name is derived from the Egyptian ḫpr "to come into existence". Khepri-Atum encompassed sunrise and sunset, thus reflecting the entire cycle of morning and evening.