His first assignment was as a beat patrolman attached to the Potrero station. In 1943, he was transferred to the Accident Investigation Bureau and, in 1946, he joined the bureau of inspectors. A year later, he was assigned to the Homicide Detail. His partner during his assignment to homicide was Inspector Francis J. Ahern. When Ahern, whose permanent civil service rank was "patrolman," was unexpectedly appointed by Mayor Roger D. Lapham to the position of Chief of the SFPD, over the heads of every captain, lieutenant, and sergeant in the department, Cahill was appointed as Ahern's deputy chief. Cahill was elevated to chief on September 5, 1958, upon Ahern's unexpected death from a heart attack on September 1, at a baseball game. As had been the case with Ahern, Cahill's permanent civil service rank was still only "patrolman." Cahill's first statement after assuming the office of chief was that Ahern's policies would continue in force, which meant "strict departmental discipline, heads up efficiency and a 'closed town.'" Colleagues remarked that Cahill "enjoyed every detail in police problems and the pursuit of justice."