He started with the band Uriel then left for college and they continued on as Egg. They reunited to record under the pseudonym Arzachel, then Hillage ended up with Kevin Ayers, then into Gong where he picked up on Daevid Allen's glissando guitar techniques that Allen had evolved from watching Syd Barrett play with a butter knife bowing strings.
Hillage became a virtuoso and the guitar sound evolved with echoes and reverb blending with synthesizers. He had a solo band after Gong and did some session work, and was a pioneer in new age ambient music and retired from recording for a while as he went on to be a prominent producer in the early 1980's, most known for his work on the Simple Minds albums of that era.
Some years later he was in a club and heard the Orb sampling his music and hooked up with them, and resurfaced with his duo with his longtime partner to record as System 7, aka 777 due to concerns from Apple over their operating system 7.
He reunited with Gong circa 2008 and toured with them in 2009 and 2010.