Parsons, along with Frank Malina, Edward Forman, and other enthusiasts, formed the GALCIT (Guggenheim Aeronautical Laboratory, California Institute of Technology) Rocket Research Group. This group would eventually evolve into the Jet Propulsion Laboratory (JPL).

The GALCIT group conducted numerous rocket experiments in the Arroyo Seco, often facing challenges with funding, safety, and the reliability of their designs. Parsons played a crucial role in developing solid rocket propellants, which were a major breakthrough.

Parsons and the GALCIT team achieved their first successful static firing of a solid-propellant rocket motor, a pivotal moment in the development of American rocketry.

During World War II, Parsons and the GALCIT team contributed significantly to the war effort by developing JATO (Jet-Assisted Take-Off) units for aircraft. These units helped planes take off from shorter runways and were vital in military operations.

The Jet Propulsion Laboratory (JPL) was officially established, with Parsons playing a significant role in its formation and initial research direction. However, his later involvement would be curtailed due to his increasingly controversial personal life.

Due to his increasingly unconventional lifestyle and involvement with Aleister Crowley's Thelema, Parsons' security clearance was revoked, leading to his departure from JPL and Aerojet. This marked a significant turning point in his career.