Guy Sion was born in 1980 in a small city on Israel's Mediterranean coastline. He started saxophone at nine, and at thirteen joined a youth band that toured Spain, Germany, the Netherlands and China — the trip to China was his first real exposure to jazz. In 1998 a scholarship to a Jazz Summer Workshop in Perugia, Italy, led to a place at Berklee College of Music; he also studied at the Rimon School of Music and was active in Israel's jazz scene before leaving the country.

He moved to Amsterdam in 2002 to study at the Conservatorium van Amsterdam, where he played for three years in the house band of the monthly "Breathing Bijlmer" event, toured with Dutch theatre actress Jetty Mathurin in 2003, and performed with the Amsterdam-based band Or Le Israel. From 2005 to 2010 he lived in New York, teaching saxophone, developing as a composer and arranger, performing with the New Amsterdam Symphony Orchestra in 2009, and releasing his debut album, Half the Battle, in 2010.

In late 2010 he moved to Oslo to teach music, and has been based there since. His band Crosscurrent, a tribute to Lennie Tristano's 1949 recording of the same name, played the Moldejazz festival in 2011. His second album, Away, followed in January 2012, recorded at Bark Studio, Oslo, and reviewed favorably by both All About Jazz and Norwegian public radio NRK P2. Since 2013 he has held the first alto saxophone chair in two Norwegian big bands, Primetime Orchestra and Ett Fett Storband, and around 2020–2021 completed a master's degree at the University of Oslo. He co-leads The Diaspora House, a project setting Ashkenazi, Sephardic and Mizrahi Jewish music in a jazz context, which has toured internationally including dates in Bucharest and at Romania's Sibiu Jazz Festival. His most recent album, We Do What We Can, was released in 2026 with a new quartet.

Sion's musical identity was shaped across four countries in sequence — Israel, the Netherlands, the United States and Norway — and his working life reflects all of them: a Lennie Tristano tribute band (Crosscurrent) sits alongside big-band section work and a project (The Diaspora House) built specifically around Jewish liturgical and folk material reworked in a jazz idiom. All About Jazz's review of Away praised him as a melodic composer, and that thread — original, tune-driven writing rather than pure vehicle-for-improvisation material — runs through both his own leader albums.