During that time, he participated in the development of high performance, inertial measurement units and flight control computers.

In 1987, Mastracchio moved to Houston, Texas, to work for the Rockwell Shuttle Operations Company at the Johnson Space Center.

An ascent/entry GPO has both pre-mission and real time Space Shuttle support responsibilities in the areas of onboard guidance, navigation, and targeting.

[4] Mastracchio was a flight engineer on Expedition 38/39 aboard the International Space Station and was one of the astronauts repairing the malfunctioning main cooling system during the mission.

The five astronauts and two cosmonauts delivered more than 6,600 pounds of supplies and installed batteries, power converters, a toilet and a treadmill.

During the mission Endeavour's crew successfully added another truss segment, a new gyroscope and external spare parts platform to the International Space Station.

On the return journey, the Leonardo Multi-Purpose Logistics Module inside Discovery's payload bay was packed with over 6,000 pounds of hardware, science results, and trash.

Mastracchio launched on Expedition 38/39 from the Baikonur Cosmodrome in Kazakhstan to the International Space Station along with Soyuz Commander Mikhail Tyurin of the Russian Federal Space Agency (Roscosmos) and Japan Aerospace Exploration Agency (JAXA) Flight Engineer Koichi Wakata.

Mastracchio launched for his fourth space flight onboard Soyuz TMA-11M alongside veteran Russian cosmonaut Mikhail Tyurin and JAXA's Koichi Wakata, with additional focus on the launch due to a publicity stunt related to the 2014 Winter Olympics, due to be held in Sochi.

On June 19, 2017, Mastracchio was appointed Senior Director of Operations for Commercial Resupply Services at Orbital ATK.