Born in Mount Sterling, Kentucky, Garrett Davis was the brother of Amos Davis. After completing preparatory studies, Davis was employed in the office of the county clerk of Montgomery County, Kentucky, and afterward of Bourbon County, Kentucky. He studied law and was admitted to the bar in 1823, whereafter he commenced practice in Paris, Kentucky.

Davis served in the Kentucky House of Representatives from 1833 to 1835. Afterward, he was elected as a Whig to the United States House of Representatives, serving from March 4, 1839 to March 3, 1847. There he was chairman of the Committee on Territories.

Davis declined to be a candidate for reelection in 1846, but instead resumed the practice of law and also engaged in agricultural pursuits. He refused to reenter politics the next fifteen years. Davis declined the nomination for Lieutenant Governor of Kentucky in 1848, and declined the American Party nomination for Governor in 1855 and for the presidency in 1856.

Davis was opposed to secession, however, and supported the Constitutional Union Party ticket in 1860. This convinced him to reenter politics, and he was elected as a Unionist in 1861 to the United States Senate to fill the vacancy caused by the expulsion of John C. Breckinridge. He was reelected as a Democrat in 1867 and served from December 10, 1861 until his death in Paris, Kentucky in 1872. He served as chairman of the Committee on Private Land Claims. He was interred in Paris Cemetery.