Mitchell Herd and Mary Edens listed as parents of Francis Marion Herd
Hawkins County, TN 1840 (text)
Lee County, VA 1850 (text) 1860 (text)
Hancock County, TN 1870 (text) 1880 (text)

Birth date of Mary Edens based on US Census records:
RECORD INDICATED AGE DERIVED BIRTH DATE
US Census 1840 under 20 After 1820
US Census 25 SEP 1850 29 1821
US Census   3 JUL 1860 37 or 39 1823 or 1821
US Census   2 SEP 1870 49 1821
US Census   1  JUN 1880 55 1825
Based on the above, I believe she was born in 1821.

Can we identify her parents?

A John Edens married Fanny Monk 28 Jan 1823
US Census 1830 Hawkins Co.,TN Page 73 A John Edens has a female member of his household in the 5-9 age bracket
US Census 1840 John Edens
US Census 1860 John Edens