The Colonel was the chosen alias of a renegade Time Lords in the Celestial Intervention Agency's employ.
The Colonel's son died in the Prydonian Academy Revolution, (PROSE: CIA File Extracts) which was also the occasion of the Doctor and the Master's flight from Gallifrey. (PROSE: Birth of a Renegade) His daughter Leoradrusendalular took him to an early retirement on Earth for safety. There, he took on the alias of "the Colonel".
He came under scrutiny by the High Council, but, as he had not been guilty of any breach of the non-intervention policy, the Council let him unmolested. The Celestial Intervention Agency contacted him shortly thereafter to make him one of their agents on Earth. To better keep abreast of events, he joined the branch of UNIT on the West Coast of North America as a scientific advisor.
A few decades into his life on Earth, he was invited back to Gallifrey as a lecturer for the Prydonian Academy. His criminological lectures on the activities of other renegade Time Lords, such as the Master, were incorporated in the CIA's databanks about the individuals in question. (PROSE: CIA File Extracts)