John Green went to MIT in 1958 for his graduate degree in electrical engineering. He joined the Instrumentation Lab in 1962 and worked in mechanical engineering on the equipment hardware. He was responsible for wiring and connecting everything for both the command and lunar modules.
Green joined up with Intermetrics after they started up in 1969, having stuck around the Instrumentation Lab for a bit after the Apollo Program ended. He continued to work in hardware, and stayed through Intermetric’s successor company, Aversatar.