Mark Cridge, who has been the managing director since he founded the business, becomes chief executive and will focus on product innovation.
Sanderson and Hagger will now take joint responsibility for the day-to-day management of the agency.
Sanderson, promoted from commercial director, will focus on the commercial and financial side of the business, including business development and making sure the agency meets the earn-out targets imposed as a result of its acquisition by Aegis.
Hagger, formerly a joint head of account management, will oversee creative output and strive to maintain glue's small-company culture as the business grows.
The other joint head of account management, Glenn Jones, becomes the sole head of account management.
The promotions mark the first time the agency has awarded senior management titles to staff other than Cridge. The business has expanded significantly this year, with staff numbers almost doubling from 46 to 80.
Cridge said the changes would allow him to spend more time on glue's creative product. "The goalposts are moving so quickly in digital that I need time to consider how the agency can adapt to new digital platforms," he said. "Glue has built its reputation on innovative creative and we need to maintain that standard."
Sanderson has been at glue for three years, having joined as its marketing director.
Before that, he worked at Smashed Atom, Telewest's interactive television company, and in business development for the digital agency DNA.
Hagger began her career at ntl Interactive, before joining glue's sister company, Deepend. She moved to glue in 2001 as an account director.