Selected as a Mission Specialist by NASA in July 2000, McArthur reported for training in August 2000. Following the completion of two years of Astronaut Candidate training and evaluation, she was assigned to the Astronaut Office Shuttle Operations Branch working technical issues on shuttle systems in the Shuttle Avionics Integration Laboratory (SAIL). She has also worked in the International Space Station and Space Shuttle Mission Control Centers as a Capsule Communicator (CAPCOM) and has served as a Crew Support Astronaut for Expedition Crews during their six-month missions aboard the International Space Station. McArthur was the Astronaut Office Lead for visiting vehicles during the first commercial cargo missions to the International Space Station. Currently, she provides support to crews in training and aboard the International Space Station, as Deputy Chief of the Astronaut Office ISS Operations Branch.