While talented in his profession, McNulty's conceited belief that he is more intelligent than his peers and his willingness to ignore the chain of command in pursuit of his own investigative projects mean that he regularly incurs the wrath of his superiors.

Before the start of the series, McNulty has noticed that drug kingpin Avon Barksdale is expanding his organization's territory, and that his gang has successfully beaten several murder prosecutions.

After Avon's nephew D'Angelo is acquitted thanks to witness tampering, McNulty goes over the head of his superior, Major Bill Rawls, and convinces Judge Phelan to call Deputy Commissioner Ervin Burrell to encourage further investigation of the Barksdales.

It initially comprises narcotics lieutenant Cedric Daniels and his three detectives: Kima Greggs, Ellis Carver, and Thomas "Herc" Hauk.

The detail is assigned as a prosecutor Assistant State's Attorney Rhonda Pearlman ("Ronnie"), with whom McNulty is having a casual sexual relationship.

Working on the Barksdale detail, McNulty becomes friends with Lester Freamon, who had previously been exiled to the pawn shop unit for 13 years and four months, as punishment for his insistence on charging a politically connected fence.

McNulty has a frank discussion with Daniels in which he admits that the Barksdale case is no more than an exercise in intellectual vanity and an opportunity to demonstrate the BPD's shortcomings.

When Rawls argues the case is not in his jurisdiction, McNulty spends three hours poring over wind and tide charts to prove the death occurred within city limits.

When port authority police officer Beadie Russell finds 13 dead women in a shipping container on the Baltimore docks, McNulty again intervenes and, with the help of the medical examiner, proves that the deaths were not accidental: the air pipe to the container was deliberately closed off, and, with the help of a mining engineer, the investigators are able to prove that the ship was within the city limits when it happened.

McNulty coerces Bubbles into tracking down Omar, who successfully testifies against Bird who is convicted of murder and sentenced to life in prison.

Meanwhile, McNulty wants to salvage his marriage, leading him to sign a separation agreement with generous alimony as a gesture of good will towards his estranged wife so she will agree to get back together.

When Daniels's unit is re-formed to investigate stevedore union boss Frank Sobotka, Rawls refuses to allow McNulty to rejoin the team, vowing he will remain working in the harbor for the rest of his career.

McNulty's first assignment is to go undercover as a client visiting a local brothel, where he scandalizes and amuses his colleagues by actually having sex with two prostitutes during the sting operation.

McNulty reasons that the time and location of the text could be used to retrieve it from the phone company's databases, and it is from this message that the detail learns the Greek had shut down his operations.

McNulty mentors Brian Baker, a younger patrol officer in the Western District, whom he and Bunk end up referring to as "good police".

[5][6] Shortly after being transferred back to Homicide, he visits the morgue, finds two county detectives arguing with the medical examiner, and subsequently learns that pre-mortem and immediate post-mortem strangulations are forensically indistinguishable.

When Landsman ignores the case, McNulty approaches reporter Alma Gutierrez of The Baltimore Sun, but only succeeds in the story's getting printed in the middle of the paper instead of on the front page.

[9][10] When McNulty finds that most dead homeless men are concentrated in the Southern District, Freamon puts him in touch with an old patrol partner there who agrees to tip them off when new bodies are found.

Freamon also devises a plan to show maturation in their serial killer's pattern, and acquires dentures to create bite marks on the victims, thus enhancing the media appeal for the story.

McNulty realizes that he can no longer falsify the murders on real corpses as there is too large a police presence, so he instead takes a mentally ill homeless man off the streets and stages a photograph of a murder to send to Sun reporter Scott Templeton, before taking the man to a homeless shelter in Richmond with fake identification.

The case is "solved" when McNulty encounters a mentally ill homeless man who has started using the modus operandi of the phony serial killer.

"[12] Also for The Guardian, Paul Owen commented that the season 1 episode "The Hunt" was the first to show "some pretty unsympathetic characteristics" of McNulty such as "sanctimonious and arrogant pronouncements".

[14] James Norton of Flak Magazine commented on how McNulty seems to fit to a standard police character archetype ("He has poor impulse control.

He's personally fearless and outspoken, and he bangs babes like a hunchback rings bells...") but ends up subverting the archetype by being self-destructive and “kind of a jerk”.

[15] Dan Kois of Salon described McNulty as "The heart, soul and oft-impaired nervous system of 'The Wire'", characterizing him as a central character.

Based on his experiences with real detectives, Simon feels that most crime dramas present their police characters with the falsehood that they care deeply about the victims in the cases they are investigating.