What ancient civilization practiced monotheism?

The only ancient monotheism was Judaism, from Abraham onward. For Jews, that is a religious truth and is stated by Maimonides based on the Torah and tradition.

• Link: How Abraham founded monotheism

What about the Ancient Egyptians or Zoroastrians? In both cases, these were monolatrous henotheistic faiths. This means that they had multiple divinities with powers not completely constrained by other divinities.
Ancient Persia:
There is no consensus on when Zoroaster lived. Zoroastrianism is unlikely to be older than Judaism, and in addition Zoroastrianism is by most standards a duotheism.
ancient Egypt:
In the case of Atenism, although all of the heavenly gods were reduced to Amun-Re, the Pharaoh was himself still held to be a god.

• Link: Were the Israelites monotheistic

Link: What do Jews believe about God?