Kallis grew up on the Mediterranean island of Cyprus, where he studied classical piano and violin while also being influenced by world music, heavy metal and jazz.

His latest scores are Disney's fantasy adventure The Last Warrior and the historical drama Cliffs of Freedom, starring Tania Raymonde, Jan Uddin, Raza Jaffrey, Patti LuPone, Christopher Plummer, Billy Zane, and Lance Henriksen.

In February 2018, George won "Breakthrough Composer of the Year" voted by the International Film Music Critics Association (IFMCA).

He also scored the Scandinavian Christmas blockbuster Julefrokosten (Fridthjof Films), starring Thure Lindhardt (Angels & Demons) and supermodel Helena Christensen.

Kallis' compositions are regularly licensed worldwide on major international and high-profile TV shows including Emmy and BAFTA-awarded programs such as The Fifth Estate on CBS, Fetch on PBS, Ultimate Fighting Championship[ (UFC), America's Most Wanted (20th Century Fox), Farah's Story (NBC/C4), and Will & Grace (NBC), to name a few.

Kallis' recent song, "The Sun Is Gonna Shine", was the theme tune for Highlander: The Source, sung by John Sloman (Uriah Heep).