Assigned to the SpaceX Dragon 2 in 2018 as part of NASA's Commercial Crew Program, Behnken launched aboard the spacecraft's first crewed mission with fellow astronaut Doug Hurley on May 30, 2020, and became one of the first two astronauts launching aboard a commercial orbital spacecraft in spaceflight history.

[5] The mission, Crew Dragon Demo-2, took Behnken and Hurley to the International Space Station (ISS), where they docked and stayed aboard for 62 days.

[8] Behnken's graduate thesis research was in the area of nonlinear control applied to stabilizing rotating stall and surge in axial-flow compressors.

During his first two years of graduate study, Behnken developed and implemented real-time control algorithms and hardware for flexible robotic manipulators.

[11] Behnken was a crew member of the STS-123 mission that delivered the Japanese Experiment Module and the Special Purpose Dexterous Manipulator to the International Space Station in March 2008.

He held the job until July 2015, when he was succeeded by Chris Cassidy, after being selected as one of four astronauts training to fly spacecraft contracted under NASA's Commercial Crew Program.

[14] Behnken and fellow crewmember Douglas Hurley were humorously compared in news and social media to the fictional brothers Bob and Doug McKenzie because of their friendship when they participated in the first commercial astronaut launch on SpaceX Crew Dragon Demo-2.

Behnken and Doug Hurley joined the ISS Expedition 63 crew, which also consisted of NASA astronaut Chris Cassidy and Russian cosmonauts Ivan Vagner and Anatoli Ivanishin.