Johnson worked in film and television as a production assistant and logger starting in 2000. . Johnson joined the crew of AMC drama Mad Men as a script co-ordinator for the second season in 2008. He became a script co-ordinator for the first series of crime drama Castle in 2008, and left the show to return for Mad Men's third season. He co-wrote the episode "The Grown-Ups" with series creator and show runner Matthew Weiner. Johnson and the writing staff won the Writers Guild of America Award for Best Drama Series at the February 2010 ceremony for their work on the third season. Johnson was also nominated for the WGA award for episodic drama at the February 2010 ceremony for his work on "The Grown-Ups" (with co-writer Weiner).