Beall launched his writing career in 2006 with the publication of his acclaimed first novel LA REX, after working for 10 years as a homicide detective and gang investigator with the Los Angeles Police Department. Beall adapted the book for Oscar-winning producer Scott Rudin and the script was featured atop The Black List, Hollywood's annual survey of the industry's favorite screenplays. Beall wrote the feature film Gangster Squad, which was released in 2013 and starred Sean Penn, Ryan Gosling and Josh Brolin. The film was based on former Los Angeles Times writer and editor Paul Lieberman's book, Gangster Squad, a non-fiction account of what he calls "the battle for Los Angeles" that took place between the police and Mickey Cohen's crew in the mid-1940s. In June 2012, Beall was hired to write the screenplay for an upcoming Justice League film, in which Superman, Batman, and Wonder Woman were to be the main characters. However, his script was later scrapped by Warners, being replaced with Academy Award winner Argo and Batman v Superman: Dawn of Justice screenwriter Chris Terrio. The studio also hired him to write an Aquaman script, but he was again replaced. In July 2016, it was revealed that he was brought back on board to handle the film's screenwriting duties, with a story treatment from James Wan and Geoff Johns. Beall was also hired by Warner Bros. to write the screenplay for a reboot of the Lethal Weapon franchise. According to IGN, Beall is writing the script to The Legend of Conan, the sequel to Conan the Barbarian.

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