The progenitors of the Lamar family in Milledgeville were the brothers John and Zachariah, who moved from Warren County, Georgia in 1810 and settled on the Putnam side of Little River, ten miles north of Milledgeville. 324 John Lamar lived in Putnam County, Georgia where he established what was to become known as the "old Lamar homestead" near Eatonton. The Lamar's 1,000 acre plantation near Milledgeville, Georgia was known as Fairfield. He had the nickname of "Little River John". John married his cousin Rebecca Lamar. The marriage produced nine children who reached maturity. Source: Florrie Belle Randle, granddaughter of Amelia Lamar Randle. With them lived his bachelor brother, Lt. Col. Zachariah Lamar, a self-taught man who, like many of the men of old plantation times, devoted himself entirely to the world of literature and history. So when sons were born to the head of the house this bookish enthusiast claimed the privilege of naming his infant nephews after his favorite character of the moment and the parents evidently consented. So we have Lucius Quintus Cincinnatus, Thomas Randolph, Mirabeau Buonaparte, Jefferson Jackson, and a grandson Lavoisier Legrand. Zachariah later married, and his daughter, Mary Ann married Howell Cobb.
On the 1820 United States Census - John Lamar, Putnam County, Georgia is on page 109. On the 1830 United States Census - John Lamar, Putnam County, Georgia is on page 174 . 352 He is the only John Lamar in the 1820-40 Putnam County census.

She was John Lamar's cousin.

Which cousin was she?