He was a member of the royal house of Lyncestis, a small Greek kingdom that had been included in Macedonia by King Philip II of Macedon.
Shortly after Philip II was assassinated by Pausanias, Leonnatus helped kill Pausanius while he attempted to flee.
After Alexander died in 323 BC, the regent, Perdiccas, made Leonnatus satrap of Hellespontine Phrygia.
In fact, Alexander promised to respect them as royalty, increase their household servants and to raise Darius' 6-year-old boy as his own.
Leonnatus led an army of 20,000 infantry with 1,500 cavalry to relieve Antipater during the siege in Lamia (see Lamian War).