The first ruler of Magadha Mahajanapadas in the sixth century BC was—

Bimbisara (c. 544-492 BCE) was the first prominent ruler of the Magadha Mahajanapada in the 6th century BCE. He belonged to the Haryanka dynasty and laid the foundation for Magadha's expansion. He was a contemporary of both Buddha and Mahavira. Ajatashatru was his son and successor. Prasenjit was the ruler of Kosala, not Magadha. Mahavira was the founder of Jainism, not a ruler.