Brock Pemberton was born on December 14, 1885 in Leavenworth, Leavenworth County, Kansas, United States. He was the son of Albert Pemberton and Ella (Murdock) Pemberton. His father, a shoemaker, was a native of Kentucky; his mother's family had migrated from Morgantown, West Virginia, to Kansas, where they pioneered in frontier journalism. He had a younger brother, Murdock, and an older sister, Ruth; the journalist Victor Murdock was his first cousin. Although reared in a strict Methodist household, Brock early developed a passion for the forbidden theater.