[1] Like another of Cheyney's characters, the FBI agent Lemmy Caution, he was constructed as a British response to the more hardboiled detectives of American fiction such as Sam Spade and Philip Marlowe.
The novels enjoyed particular popularity in France[2] where actor Tony Wright played Callaghan in three film adaptations.
Two West End stage versions in the early 1950s Meet Mr. Callaghan and Dangerous Curves by Gerald Verner starred Terence De Marney.
Operating out of an office in Mayfair's Berkeley Square he frequently encounters attractive, but deceitful femmes fatales.
Callaghan has been described as a "suave, handsome, resourceful, Mayfair-based private detective who is Irish – the nationality was perhaps a tease for Cheyney's patriotic critics".