McCullers started his career as Writer on Saturday Night Live. He later co-wrote the scripts for Austin Powers: The Spy Who Shagged Me and Austin Powers in Goldmember, two films of the Austin Powers film series starring Mike Myers, which became his most acclaimed works. He later wrote and directed the 2008 comedy Baby Mama starring Tina Fey and Amy Poehler.
McCullers later went to write the 2017 film The Boss Baby for DreamWorks Animation. He had recently finished the scripts of Hotel Transylvania 3 for Sony Pictures Animation along Genndy Tartakovsky with a release date in 2018 and Shrek 5 for DreamWorks Animation.