He is a veteran of three spaceflights to the International Space Station and holds the record for the oldest person to perform a spacewalk at 61 years old.

During this time, he also worked as an attending physician for the emergency medicine residency at The University of Texas Health Science Center at Houston.

[1] Marshburn joined NASA in November 1994, serving as a flight surgeon at Johnson Space Center in Houston, Texas.

[1] After spending ten months as a NASA representative to the Harvard/MIT Smart Medical Systems Team of the National Space Biomedical Research Institute in Boston, Massachusetts, he worked as the lead Flight Surgeon for Expedition 7 to the ISS in 2003, supporting from Russia, Kazakhstan and Houston.

His activities included development of the biomedical training program for flight surgeons and astronaut crew medical officers, and managing the ISS Health Maintenance System.

The crew was welcomed aboard the ISS by Expedition 34 commander Kevin A. Ford and cosmonauts Evgeny Tarelkin and Oleg Novitskiy.

[1] On May 11, 2013, Marshburn and Expedition 35 flight engineer Christopher Cassidy performed an unplanned spacewalk to replace a pump controller box suspected to be the source of an ammonia coolant leak.

[1][10] On November 11, 2021, Marshburn launched on the SpaceX Crew-3 spaceflight as part of the long duration Expedition 66 mission onboard the ISS.

[13] After leaving NASA, Marshburn joined Sierra Space in late 2022 as the Chief Medical Officer in the company's Human Spaceflight Center and Astronaut Training Academy.