Seddon became an astronaut on August 9, 1979, after selection as a candidate the year prior. At NASA her development work included the Space Shuttle Orbiter and payload software, the Shuttle Avionics Integration Laboratory, the Flight Data File, the Space Shuttle medical kit, and checklists for launch and landing. She was a rescue helicopter physician for the early Space Shuttle flights and a support crew member for STS-6. She served as a member of NASA's Aerospace Medical Advisory Committee, as a technical assistant to the director of flight crew operations, and as a capsule communicator (CAPCOM) in the Mission Control Center. In 1996 she was detailed by NASA to Vanderbilt University Medical School in Nashville, Tennessee, where she assisted in the preparation of cardiovascular experiments that flew on the STS-90 Neurolab Spacelab flight in April 1998. She retired from NASA in November 1997 and became Assistant Chief Medical Officer of the Vanderbilt Medical Group.
