George St.John is listed as an early settler of Cannon Co., Tenn., formed in 1836 (Tenn. Cousins, by Worth S. Ray). I realize this is earlier than the George you're interested in, as this George was born in 1765 and died in 1855 in Cannon Co. According to family tradition, he married first Evaline Jolly and they had two children, a son John Isaac and a daughter Evaline. This might give you a place to start.
Also, check out Clifton's Notes (re: St.Johns) on the Web. He has more information further down the line, I believe.