Clarence G. Jackson was born on March 25, 1842 in Berwick, Pennsylvania. He was one of the sons of self-made heavy manufacturer M. W. Jackson and his first wife, Margaret Gearhart Jackson. The younger Jackson grew up in Berwick and at fourteen attended the Dickinson Seminary in Williamsport. He then enrolled in Dickinson College, Pennsylvania at the age of sixteen with the class of 1860. He was elected to the Belles Lettres Society and graduated with honors along with his class.
Jackson returned to Berwick to work with his father and study law, but enlisted in August 1862 as a second lieutenant in Company H of the Eighty-fourth Pennsylvania Volunteer Infantry. The newly raised regiment organized at Camp Crossman, near Huntingdon, and went on to participate in some of the heaviest fighting of the war. Jackson was promoted to first lieutenant in January 1863 and then was wounded and captured at the Battle of Chancellorsville on May 3, 1863. He was imprisoned at the infamous Libby Prison in Richmond before being exchanged. Returning to the 84th, Jackson was promoted and took over command of Company H. He was captured again at the Battle of the Wilderness and returned to Libby, before being transferred as one of the 600 officers sent to Charleston allegedly to serve as "human shields" against the Union shelling of the city. He was once again exchanged and served until the end of the war in Company H, which by that time had been amalgamated in January 1865 with the Fifty-Seventh Pennsylvania.
Jackson returned to Berwick and his father's businesses in 1865, but also remained involved in military affairs. He was appointed as a major in the newly organized Pennsylvania National Guard in 1870. He was promoted to colonel on the governor's staff during Governor Hartranft's revamping of the Guard in the late 1870s and ultimately was appointed as quartermaster general of the organization in 1879. By this time, he was a vice-president in the Jackson and Woodin Manufacturing Company and a director of the First National Bank of Berwick. Jackson served as the school director in Berwick and trustee of the local Methodist Episcopal church. He was also a trustee of his alma mater from 1875-1880. On his death, Jackson's family donated $10,000 to the Dickinson College endowment in his memory.
Upon returning from the war, Jackson married Elizabeth Seybert of Berwick in 1866. The couple had two daughters. Clarence Gearhart Jackson died unexpectedly at his new palatial mansion in Berwick on May 13, 1880. He was thirty-eight years old.