[4] Monahan was an editor at Spy during the magazine's final years, where he would come in at the close of the monthly issue to rewrite articles and improve jokes.

"[6][7][8] In the second half of 2001 Monahan wrote a fictional column at the New York Press under the pseudonym of Claude La Badarian, which ran for 13 weeks.

Brad Pitt's production company, Plan B, hired Monahan to write an adaptation of Hong Kong director Andrew Lau's gangster film Infernal Affairs.

Monahan respun Infernal Affairs as a battle between Irish American gangsters and cops in Boston's Southie district, and Martin Scorsese directed the completed screenplay under the title The Departed for Warner Bros.[17][18] Monahan's work on the film would later earn him two Best Adapted Screenplay awards, from the Writers Guild of America and the Academy Awards.

Kingdom was critically reappraised when it was released on DVD in the form of a director's cut that contained an additional 45 minutes of footage previously shot from Monahan's shooting script.

"[13][20] Monahan received considerable praise from critics when the film was released in theaters, in 2006, and was applauded for accurately depicting the city of Boston.

[22][23][24] Monahan took an unusual route for a screenwriter and hired a publicist to run a campaign promoting his screenplay during awards season.

In return Henceforth received the film rights to produce John Pearson's true crime book The Gamblers, a property which Warner Bros. had previously acquired.

Monahan was initially assigned to executive produce and write the adaptation for Confession of Pain, under production by Leonardo DiCaprio's company, Appian Way, for Warner Bros.

Martin Scorsese became involved while the film project was at Disney and subsequently negotiated a turnaround deal to bring The Long Play to Paramount.

[31] Others adjudged the film as unfocused, complaining of "a surplus of plot threads that don't have space to play out, and accordingly com[ing] across as clichés",[32] that he "ended up with more than he can chew for his first time in the director's chair".

[47] The Rotten Tomatoes consensus for the movie is that it "has no shortage of talent on either side of the camera; unfortunately, it amounts to little more than a frustrating missed opportunity.

[49] The Observer's Rex Reed labeled it "gibberish with guns and phony literary pretentiousness about two thugs in a duel of weapons and words that goes nowhere fast", contending that his high-quality work on The Departed was inexplicable, as he had "written nothing of value since".