John was probably the fourth child of Thomas Lamar II. He sold his inherited lands in Maryland in 1755 and with his brothers moved to South Carolina (Beech Island in the Savannah River). He received a grant of 300 acres of land on Beaver Dam Creek in 1757, but in 1759 he moved to Georgia where he received a grant of land adjoining that of his brother Alexander Lamar. At the time of his arrival in Georgia, one of his children still lived at home, the other sons being evidently married and away from home. The time of his death is not known, although he is known to have been alive in 1771 when he executed a deed correcting a mistake in the conveyance of the Maryland lands which he had sold in 1755.

Source: "History of the Lamar or Lemar Family in America," Harold Dihel LeMar, 1941

John might possibly have had a daughter named Martha who married Nathaniel Coates and had a daughter Jane Smiley Coates in 1760. Nathaniel Coates was the High Sheriff in Wilkes County, GA. Martha was supposed to be from the "prominent Lamar family of Georgia". Richard Abell, a Federal Judge who authored a book on A. P. Adamson, grandson of Martha and Nathaniel states "Genealogists have concluded that Martha Lamar was the daughter of John Lamar (1713-1785) and his wife Rachel."

Was there another son named William?