Armstrong joins from Agency Republic, which recently merged with Critical Mass, where he was business director. As part of the senior management team, he led the Vodafone, Nivea and PepsiCo accounts.
He joins The Red Brick Road next week as head of digital and social media.
Armstrong has 15 years' experience in agencies and has worked on a variety of brands including Aviva, Yell.com and BMW.
David Miller, TRBR’s managing director, said, "Stephen will bring years of experience delivering digital platforms and strategies that transform how brands perform, and will be key in driving forward TRBR’s integrated offering."
The Red Brick Road recently won the creative account for Broadbandchoices.co.uk after a three-month pitch process.