MacMullan joined Pixar Animation Studios in 2009 to work on the feature Newt, which was soon cancelled. She them moved to the Disney studio and became a storyboard artist on Wreck-It Ralph. While working on the film, her friend and former Simpsons colleague Rich Moore told her that Disney was looking for new Mickey Mouse ideas for television.
Since she was always fascinated with the late 1920s/early 1930s era Mickey shorts, she conceived and directed the theatrical short Get A Horse!, which earned her an Academy Award nomination.
Prior to joining Disney, she worked as a director on numerous animated TV series, including The Critic, King of the Hill, The Simpsons, and Avatar: The Last Airbender. She was also the supervising director and designer for the series Mission Hill, and was a sequence director on the animated feature The Simpsons Movie.