Gaillard was born in St. Stephen's district, South Carolina, on September 5, 1765.

He was elected to the United States Senate in place of Pierce Butler, who resigned, and served from January 31, 1805, until his death in Washington, D.C., on February 26, 1826.

He was also first in the presidential line of succession from November 25, 1814, two days after the death of Vice President Elbridge Gerry, to March 4, 1817.

In his memoir Thirty Years' View, Thomas H. Benton, one of Gaillard's contemporaries, described him thus: Urbane in his manners, amiable in temper, scrupulously impartial, uniting absolute firmness of purpose with the greatest gentleness of manners—such were the qualifications which commended him to the presidency of the senate.

He classed democratically, but was as much the favorite of one side of the house as of the other, and that in the high party times of the war with Great Britain, which so much exasperated party spirit.Gaillard died in Washington, D.C., on February 26, 1826, and was interred in the Congressional Cemetery.