He was said to have been the son of Belus or Bel, a name that may be a Semitic title such as Ba'al, "lord". He was reputed to have conquered the whole of western Asia in 17 years with the help of Ariaeus, king of Arabia, and to have founded the first empire (sometimes referred to as pre-Babylon, or simply Mesopotamia).
Many early accomplishments are attributed to him, such as training the first hunting dogs, and taming horses for riding. For this accomplishment, he is sometimes represented in Greek mythology as a centaur.
During the siege of Bactra, he met Semiramis, the wife of one of his officers, Onnes, whom he took from her husband and married. The fruit of the marriage was Ninyas, i.e. "The Ninevite". This child is sometimes referred to as Tammuz .
He has sometimes been thought to be connected with the figure identified in the Torah as Nimrod the hunter", son of Cush. In some interpretations of the Hebrew text of Genesis 10, it is Nimrod who founded Nineveh, but this too is ambiguous: other translators render the same Torah verse as naming Ashur, son of Shem as the founder of Nineveh.
After the death of Ninus, Semiramis, who was accused of causing it, erected to him a temple-tomb, 9 stades high and 10 stades broad, near Babylon, where the story of Pyramus and Thisbe was later based. According to Castor of Rhodes (ap. Syncell. p. 167), his reign lasted 52 years, its commencement falling in 2189 BC according to Ctesias.
The story of Ninus and Semiramis is taken up in a different form in a 1st century AD Hellenistic romance called the Ninus Romance, the Novel of Ninus and Semiramis, or the Ninus Fragments. A scene from it is perhaps depicted in mosaics from Antioch on the Orontes
Another Ninus is described by some authorities as the last king of Nineveh, successor of Sardanapalus.