He joins the agency early next month and will report to the executive creative director, Russell Ramsey.
His appointment comes just two months after JWT also hired the former MCBD creative Jeremy Carr to take on a similar creative director role.
Masterman began his advertising career working on Volkswagen at BMP DDB in 1994, before moving to Fallon to handle accounts including Skoda, Ben & Jerry's, Sony and the BBC.
In 2004, he joined Bartle Bogle Hegarty, where he created the Lynx "click" campaign featuring Ben Affleck and also worked with Ramsey on the agency's Audi account.
In 2006, CHI hired Masterman as a creative director to oversee brands including Tiger Beer, Tango and Drench. He also created the "Great Escape" animated ad for Anchor, while his "cubehead" cinema spot for Drench received a bronze Film Craft Lion at last month's Cannes Festival of Creativity.
For the bulk of his career, Masterman has worked as part of a creative team with Ed Edwards, who recently departed CHI to join the boutique agency Devilfish as its first ECD.
Masterman said: "JWT has a client list to die for, so I really can't wait to get started."