Robert Ward Johnson was born in Scott County, Kentucky. He attended Choctaw Academy and St. Joseph's College in Bardstown, Kentucky. He moved with his father to Arkansas in 1821.
Johnson studied law and was admitted to the Arkansas bar in 1835. He was elected prosecuting attorney for Little Rock, Arkansas and served from 1840 to 1842 and effectively acted as the state's attorney. Johnson took up residence in Helena, Arkansas prior to the Civil War.
Johnson was elected to the Thirtieth, Thirty-first, and Thirty-second congresses. He was chairman on the Committee on Indian Affairs.
Johnson declined to run for reelection in 1852. He was appointed and later elected to the United States Senate to fill the unexpired term of Senator Solon Borland. He was reelected in 1855 and served until 3 March 1861.
After the outbreak of the American Civil War he served as a delegate to the Provisional Government of the Confederate States in 1862. Member of the Confederate Senate from 1862 to 1865.
After the war he practiced law in Washington, D.C. and ran unsuccessfully for reelection to the Senate in 1878.
Robert Ward Johnson died in Little Rock, Arkansas. Johnson is buried in the historic Mount Holly Cemetery in Little Rock.
Robert Ward Johnson was the nephew of Vice President of the United States Richard Mentor Johnson and his brothers James Johnson and John Telemachus Johnson who were both US Representatives from Kentucky. Robert Ward Johnson was the brother-in-law of Senator Ambrose Hundley Sevier, Sevier marrying Johnson's sister. Johnson himself married twice, first to Sarah Smith in 1836, whom he had six children with (three living to adulthood), and, after Sarah's death in 1862, her younger sister, Laura, in 1863, with whom he had no children.