Augustus Hill Garland was born in Covington, Tennessee. His family moved to Hempstead County, Arkansas in 1833. Garland attended St. Mary's College in Lebanon, Kentucky and graduated from St. Joseph's College in Bardstown, Kentucky in 1849. Garland studied law and was admitted to the Arkansas bar in 1853. He opened a law practice in Washington, Arkansas and practiced there until moving to Little Rock, Arkansas in 1856. In 1860 he was admitted to practice law before the United States Supreme Court.

Garland served as a presidential elector for the Constitutional Union Party in the 1860 election. Garland was a pro-Union delegate to the Arkansas Secession Convention in 1861. He was a member of the provisional Confederate Congress that met in Montgomery, Alabama in May of 1861 and served in the Confederate House of Representatives from 1861 to 1864 and the Confederate Senate for the last two years of the war.

At the end of the war Garland was stripped of his law license by the U.S. government and received a pardon from President Andrew Johnson in the summer of 1865.

Garland made legal history as the subject of the Ex parte Garland case of 1867 which was a U.S. Supreme Court case in which he argued that the act barring ex-members of the Confederate government from practicing law was an ex post facto law and was thus unconstitutional. The court ruled in favor of Garland and the proscriptions on his legal career were removed. This ruling caused considerable uproar in the north.

He was elected to the United States Senate for the term beginning in 1867 but was not allowed to take his seat.

He served as Governor of Arkansas from 1874 to 1876. During his tenure he created the Branch Normal College which trained African-American teachers. He advocated for financial support of the schools for the blind and deaf. Garland also dealt with the serious financial problems of the state.

In 1876 he was elected once again to the United States Senate. He was reelected in 1883 and served until resigning on March 6, 1885 to become the Attorney General of the United States under President Grover Cleveland. He served as Attorney General until 1889.

After leaving office he resumed the practice of law in Little Rock and published several books including Experiences in the Supreme Court of the United States and A Treatise on the Constitution and Jurisdiction of the United States Courts.

Garland died in Washington, DC inside the United States Capitol while arguing a case before the Supreme Court. He is buried at historic Mount Holly Cemetery in Little Rock.

Garland County, Arkansas and Garland, Texas are named after Augustus Hill Garland.