Leonnatus was born in Pella, a member of the royal house of Lyncestis, a small kingdom that had been included in Macedonia by King Philip II of Macedon. He was the same age as Alexander and was very close to him. Later, he was one of Alexander's seven bodyguards, or "somatophylakes".
After the Battle of Issus, Leonnatus was sent to the captured family of Darius to reassure them of Alexander's benevolent intentions towards them.
He was present when Alexander killed Clitus the Black in a drunken fit and saved Alexander from suicide when the latter repented of his action.
Alexander's sister Cleopatra, the widow of King Alexander I of Epirus, offered her hand to Leonnatus. Meanwhile the Athenians heard that Alexander had died and revolted against Macedonia and the new regent, Antipater. Leonnatus led an army of 20,000 infantry with 1,500 cavalry to relieve Antipater during the siege in Lamia (see Lamian War). He was killed in battle against the Athenians and his marriage with Cleopatra never took place.