Lamar was born in 1798 in Louisville, Georgia,he and grew up at Fairfield, his father's cotton plantation near Milledgeville, then the state capital. His father's family was descended from French Huguenot Thomas Lamar, who had settled in Maryland in 1660. His parents, John and Rebecca (Lamar) Lamar had allowed his mother's brother to name their sons; he named them after his favorite historical heroes. The elder brother was named for the Roman statesman Lucius Quinctius Cincinnatus; the younger, Mirabeau Buonaparte Lamar for French heroes. The family had connections with other families throughout Georgia and the South. As a child, Lamar loved to read and educated himself through books. Although he was accepted to Princeton University (then called the College of New Jersey), he chose not to attend. He started work as a merchant and then ran a newspaper, but both of those enterprises failed.