BALTIMORE—Edwin “Ed” Hale Sr., a retired bank executive known locally for his sharp-elbowed approach to business, installed video surveillance on his 186-acre farm and still sleeps with a sawed-off shotgun by his bed.

His friends, former employees and even his own daughters were shocked to learn in his recently published biography that he had ample reason to do so: The former chief executive and chairman of Bank of Baltimore says he worked covertly for the Central Intelligence Agency for almost a decade in the 1990s and early...