Leonnatus was the son of Anteas, a member of the royal house of Lyncestis, a small kingdom in the valley of the Crna that had been included in Macedonia by king Philip, the father of Alexander and the son of a mother who belonged to the Lyncestian dynasty.
Leonnatus, being a distant relative of Alexander, must have been close to the crown prince. They were of the same age and may have shared the same teachers (e.g., the famous Macedonian philosopher Aristotle of Stagira). The Lyncestian prince was present when Philip was assassinated in October 336 and played a role in the pursuit and killing of the killer.
When Alexander set out against Persia, Leonnatus was in his personal company. Military commands are unknown, but he played a role in the aftermath of the battle of Issus (November 333). The Persian king Darius III Codomannus had been defeated, his chariot and bow captured, and his mother, wife, and daughters taken prisoner. When the women saw the bow and chariot, they thought that Darius was dead, and started mourning. Alexander sent Leonnatus to inform them that Darius was not dead, and that they need not fear any harm (text).
When Alexander was in Egypt, one of his seven bodyguards died, and Leonnatus was appointed as successor. This meant that he was very close to the king and could exercise great influence. He could make and break people, as Philotas was to discover in October 330: the cavalry officer was suspected of treason and Leonnatus was one of his accusers. He was executed. On the other hand, when Alexander was angry with the aged officer Clitus and wanted to kill him, Leonnatus tried to prevent the murder. However, this time, he had insufficient influence on his king (Autumn 328). Later, he discovered a conspiracy among the pages.
There is a strange story that Leonnatus was so fond of wrestling, that he took trainers and dromedary-loads of sand wherever he went in Asia. If there is any truth to this, we must date it in these years.
In the Summer of 327, Alexander wanted to introduce the Persian custom of proskynesis at his court. Most Macedonians felt offended by what they considered to be an act of slavish submissiveness. Leonnatus even started to laugh and make fun of the ritual, which caused Alexander's anger. They were reconciled, however, and in the months after the incident, we meet Leonnatus for the first time as a commander: with Ptolemy (another bodyguard) and Perdiccas (a cavalry officer), he besieged one of the mountain fortresses in Sogdia. In the Spring of 326, he played a role in the attack on a village in Gandara (i.e., the valley of the river Kabul), where he was wounded. The wound cannot have been very severe, because in May, he took part in the battle of the Hydaspes, in which Alexander defeated the Indian king Porus.
That Leonnatus, in spite of the fact that he did not command large units, remained very close to Alexander, can be shown from the fact that he was one of those who occupied the office of trierarch in the Autumn of 326. They were responsible for the building of parts of the fleet that the Macedonians had to build to advance along the Indus river to the Indian Ocean. On their voyage to the south, they had to subdue the Mallians, which proved to be difficult. During the siege of their capital, modern Multan, Alexander was seriously wounded by an arrow; he owed his survival to his bodyguards Leonnatus, Abreas and Peucestas, who protected the king with the sacred shield that the king had taken with him at Troy (January 325).
In July, the army reached the Ocean. It was divided in two groups: Alexander commanded an army that moved along the coast to the west to Carmania, and Nearchus led the navy. Alexander's men first defeated the Oreitans, the "mountain people of southern Pakistan. Hephaestion, who commanded the baggage train, was ordered to build a new city, Rhambaceia (modern Las Bela). Leonnatus was left behind to defend the region; after all, Nearchus still had to pass along the shore and the grain store had to be kept intact. The Oreitans attempted to attack the Macedonian rearguard, but Leonnatus defeated them, for which he received a diadem in the spring of 324, when the armies had reunited at Susa (text).
On 11 June 323, Alexander died in Babylon. Immediately, his generals - known as the Diadochi - started to quarrel about the succession, in which cavalry and infantry were at each other's throats. Leonnatus sided with Perdiccas, Lysimachus (another bodyguard), Seleucus, Eumenes and Ptolemy, cavalry commanders that were to play an important role in the next years. The commander of the infantry Meleager was killed, and Perdiccas was appointed regent for the new king, Alexander's mentally unfit brother Philip Arridaeus. Perdiccas made Leonnatus satrap of Hellespontine Phrygia.
A civil war was prevented, but war had become inevitable. When the Athenians heard that Alexander had died, they revolted (Autumn 323). They had been preparing the war for some time and were joined by several other Greek towns. They occupied Thermopylae, and when the Macedonian commander Antipater arrived, he was repelled and forced to hide in the nearby fortress of Lamia.
In the meantime, Alexander's sister Cleopatra, the widow of king Alexander of Molossis, offered her hand to Leonnatus. If they married, Leonnatus would be a powerful rival to Perdiccas, and might reasonably claim the throne. However, the marriage never took place. In the Spring of 322, Leonnatus set out from his satrapy to relieve Antipater. A victory over the Greeks would certainly enhance his claim to the throne. He was successful, but was killed in action, having survived his friend with only one year. It is tempting to think that he was murdered to prevent a civil war with Perdiccas.