Mary McMillin was the daughter of James Benton McMillin Sr. and Katherine Halsell McMillin. This name also appears as McMillen and McMillan in different documents. Her father is described as, "... a gentleman of Scotch blood, and for several terms a member of the Kentucky legislature, noted for his good humor, anecdote, and hospitality," in the source, "Prominent Tennesseans." This document included a profile of her husband, Judge E.L. Gardenhire. Her mother Katherine Halsell was of Irish descent.

"Prominent Tennesseans," compiled by William S. Speer in 1888, provides the best insight into Mary Gardenhire. It states, "Mrs. Gardenhire was educated at Centre Point, Kentucky, and, like her family generally, is very popular; an intellectual woman, with a fine sense of the ludicrous, and possessed of that agreeable vivacity and witty turn that intelligent Irish people generally have."

Mary McMillin married Erasmus Lee Gardenhire, son of Adam and Ailsey Tippet Gardenhire on 5 Dec 1839 in Overton Co. , Tn. They were the parents of 8 children:
- James Alexander "Alexis"
- Alice Catherine
- John Halsell
- Mary Catherine
- Lucy Ellen
- Ada M.
- Rassalee
- Erasmus Lee Jr.

Mary McMillin Gardenhire is found in the following US Federal Census records:
- 1850 for Overton Co., Tn. with spouse, Erasmus Lee Gardenhire, and two sons and two daughters
- 1860 for White Co., Tn. with spouse and seven children
- 1870 for White Co., Tn with spouse and seven children
- 1880 for Smith Co., Tn with spouse and three children

We know from her newspaper obituary in the Southern Standard, McMinnville, Tn., Mary McMillin Gardenhire died suddenly in Sparta, White Co., Tn, at the home of her daughter, Lucy Ella Gardenhire Quarles. Her burial is in Sparta remains unconfirmed.

Perhaps she is found listed as M.H. buried in the Gilliland Farm cemetery in the source, "Overton County Roll Call, Cemetery Records 1795-1995," by Gary Denton Norris, copyright 1987. At this time, this match remains unproven. This cemetery is within sight of the home her husband, Erasmus Lee Gardenhire, grew up in, and her youngest son, E.L. Gardenhire Jr. is believed to be buried there.

Much of what we know about Mary McMillin Gardenhire comes from a variety of sources:
- The Gardenhire family bible gives her date of birth
- "Know your Legislators" selected Sketches of Biographical Directory of the Tennessee General Assembly, compiled from several sources
- "The Southern Gardenhires" by Lawrence W. Gardenhire
- the memoir of her husband, Erasmus Lee Gardenhire, titled a Brief Sketch of My Life, also found in "The Southern Gardenhires document

- compiled by Gaynelle Hobt Morton
- Sept 2017
- revised May 2019
- revised Aug 2023