It was Sony ATV Amanda Berman-Hill, who suggested that Maren Morris write with Sarah Aarons and Greg Kurstin. Berman-Hill is both Aarons' publisher and Kurstin's publisher and after the success of "The Middle" Morris wanted to write more with Aarons. The singer explained to Billboard
that Berman-Hill told her, "Sarah has written a bunch with Greg - you should do a trio and get in a room together and see what happens."
The trio were paired together for a couple days and they came up with "Girl" on the first day.