Buchanan was born in Midway, Orangeburg County, South Carolina, on April 30, 1867; later that year, his family moved to Texas and settled near Chappell Hill in Washington County.

He attended the local schools of Chappell Hill, and in 1889 he received his law degree from the University of Texas at Austin.

Later in 1889, Buchanan began a law practice in Washington County, and almost immediately became involved in politics as a Democrat.

In 1913, he was elected to the Sixty-third Congress, filling the vacancy left by the resignation of Albert Sidney Burleson.

Buchanan was elected to a full term in the Sixty-fourth Congress, and won reelection eleven times.