Montgomery, a former Wieden & Kennedy Amsterdam creative, joined Grand Union a year ago. It was his first role in a digital agency.
James Woods and Stuart Hallybone, a senior creative team at the agency, will replace Montgomery as joint creative directors and take responsibility for creative output across clients including Stella Artois and COI.
Meanwhile, Paul Sonley, a senior designer, will take the role of design director, overseeing the agency's web development work.
Woods, Hallybone and Sonley will report to the newly appointed chief executive, Anna Watkins, and managing director, Jaimes Leggett, who were promoted as part of a management restructure.
Rob Forshaw, founding partner at Grand Union, said: "Giles is a great talent and brought a lot to the agency in his short time here. However, both sides felt that it wasn't quite working out."