Cary was a member of the House of Burgesses from 1756 to 1776. In 1764, he served on the committee of Burgesses that wrote resolutions against the proposed Stamp Act, but the following year he voted against Patrick Henry's Virginia Resolves as being premature and too inflammatory.
As tensions with the mother country escalated, in 1773 Cary served as a member of Virginia's committee of correspondence. When the House of Burgesses was dissolved at the outset of the American Revolution, he served as a delegate to the Virginia Conventions. At the Virginia Convention in May 1776, he served as the chairmen of the committee of the whole that adopted the celebrated resolution of independence, which instructed Virginia's delegates to the Second Continental Congress to propose a declaration of independence. After Virginia became an independent state in 1776, Cary became the first speaker of the Senate of Virginia, and remained in that position until his death.
During the American Revolutionary War, Cary was placed in charge of recruitment and supplies in central Virginia. He was asked by Thomas Jefferson, his colleague in the House of Burgesses and fellow graduate of William and Mary College, to loan the Virginia Colony the funds to underwrite the cost of the Virginia militia, on the promise by Jefferson he would be repaid later, though he never was repaid. He did fund the Virginia militia for the following reason: though he had always been loyal to the Crown (he had a Charter from the Crown for all his thousands of acres of property at Ampthill plantation), he had grown tired of British attempts to continue promoting the sale of slaves in America. Although he owned some 200 slaves, he had come to the conclusion that everything about the slave trade and the owning of slaves was only going to create major problems.
On May 31, 1744, Cary married Mary Randolph, the daughter of Richard Randolph of Curles. Through this marriage, his children were lineal descendants of Pocahontas. One of their daughters, Anne, married Thomas Mann Randolph. In 1768, Cary's daughter, Jane, married Thomas Isham Randolph, the son of Isham Randolph and an uncle of United States President Thomas Jefferson. Cary was known among Baptists for arresting many Baptists for preaching without a license. There was one incident where a Baptist preacher continued to preach from his cell window. To solve the problem, Cary put a wall around the prison.
His nickname was "Old Iron". He operated Chesterfield Forge, which fabricated iron, starting in 1750, and ending in 1781, when it was burned by Benedict Arnold. He owned British thoroughbred horses and traded with England.
- Tyler, Encyclopedia of Virginia Biography, 8.
- Virginia. General Assembly, Journals of the House of Burgesses of Virginia, 1766-1769, p.11
- Page, Richard Channing Moore (1893). "Randolph Family". Genealogy of the Page Family in Virginia (2 ed.). New York: Press of the Publishers Printing Co. pp. 249–272.
- Tilton, Robert S. (1994). "Notes". Pocahontas: The Evolution of an American Narrative. Cambridge University Press. p. 191. ISBN 978-0-521-46959-3.
- Glenn, Thomas Allen, ed. (1898). "The Randolphs: Randolph Genealogy". Some Colonial Mansions: And Those Who Lived In Them : With Genealogies Of The Various Families Mentioned 1. Philadelphia, Pennsylvania: Henry T. Coates & Company. p. 459.
- Falling Creek History, The Falling Creek Ironworks Foundation
- Brock, Robert K. Archibald Cary of Ampthill: Wheelhorse of the Revolution. Richmond, Virginia: Garrett and Massie, 1937.
- Little, Lewis Peyton. Imprisoned Preachers and Religious Liberty in Virginia. Lynchburg, Virginia: J.P. Bell Co., 1938.
- Tyler, Lyon Gardiner, ed. Encyclopedia of Virginia Biography, volume 2. New York: Lewis Historical Publishing, 1915.