From his debut, Callahan became the template for a new kind of film cop: an antihero who does not hesitate to cross professional boundaries in pursuit of his own vision of justice, especially when the law is poorly served by an inept, incompetent bureaucracy.

[1] Callahan is an Inspector with the San Francisco Police Department, usually with the Homicide Division, although for disciplinary or political reasons he is occasionally transferred to other less prominent units, such as the Personnel Division (in The Enforcer) or Stakeout (in Magnum Force) or just sent out of town on mundane research assignments (in Sudden Impact).

In The Dead Pool, Callahan shoots a fleeing and unarmed Mafia assassin in the back and kills the villain in the end with a harpoon knowing that the man's pistol is out of ammunition.

He often clashes with superiors who dislike his methods, and judges and prosecutors are wary of handling his cases because of frequent violations of the Fourth Amendment and other irregularities.

Callahan for his part believes his superiors have got their priorities in the wrong place, seemingly worried too much about the department's reputation, and not enough about actual security.

A police commissioner admits that Callahan's "unconventional methods ... get results", but adds that his successes are "more costly to the city and this department in terms of publicity and physical destruction than most other men's failures".

In Sudden Impact he is threatened with a transfer to Traffic and being fired, in The Enforcer he begins a 180-day suspension imposed by McKay, and in The Dead Pool he is only allowed to stay off desk duty with a new partner.

[3] The films routinely depict Callahan as being a skilled marksman and strong hand-to-hand combatant, killing at least one man with his bare hands.

In the five films, Callahan is shown killing a combined total of 43 criminals, mostly with his trademark revolver, a Smith & Wesson Model 29 .44 Magnum, which he describes as "the most powerful handgun in the world".

He refuses to join the secret police death squad in Magnum Force, as he prefers the present system, despite its flaws, to the vigilante alternative.

In his fight against criminals, however, including the fellow officers on the death squad, Callahan is relentless and shows no hesitation if he has to use ultimate force.

When his partner Chico Gonzalez asks about the nickname's origins, Frank DiGiorgio says, facetiously, that "that's one thing about our Harry; [he] doesn't play any favorites.

In Dirty Harry, Gonzalez humorously suggests that Callahan's nickname may have an alternate origin given that he twice ends up peeking through a naked woman's window and later follows a suspect into a strip club.

In The Dead Pool, a coffee mug on Harry's desk at the police station bears the United States Marine Corps seal and in The Enforcer it is clear he has already been checked out on the LAWS rocket, a USMC weapon.

He appears to subsist on a diet of hot dogs, hamburgers and strong black coffee which he takes without sugar and is so unchanging that he simply orders 'the usual' from the staff of his regular eateries (in The Dead Pool he samples his girlfriend's unknown dessert but does not have one himself).

More than once Callahan resents the partners assigned to him: Gonzalez because he is a rookie college graduate, and Moore because she is a woman lacking field experience.

In The Dead Pool, he implies a romantic relationship with news reporter Samantha Walker, and the film ends with her and Callahan walking away together.

Callahan is considered a film icon, so much so that his nickname, "Dirty Harry", has entered the lexicon as slang for ruthless police officers.