Colonel Johnson is the Chief Constable who appears in several Agatha Christie books.
In Three Act Tragedy (1934), he is described as a "big red-faced man with a barrack-room voice and a hearty manner". He is a friend of Mr Satterthwaite, and is delighted to meet Sir Charles Cartwright, as his wife is a great playgoer. Colonel Johnson tells them what the police have found in the investigation of the death of Sir Bartholomew Strange, including their suspicion of the butler, Ellis, who disappears the morning after the murder.
Colonel Johnson appears again in Hercule Poirot's Christmas (1938). That this is the same person is demonstrated by the fact that he mentions "that Cartwright case" to Poirot.
In Three Act Tragedy, it is stated that his county is Yorkshire. In the Hercule Poirot's Christmas, written 4 years later, his county is stated as "Middleshire" (a fictional county name). This could be a new county or Yorkshire under a pseudonym.