Horrobin has been at the Wardour Street post house for nearly 15 years.
His credits include promos for Channel 4's The Big Food Fight and teasers for BBC1's Hustle and Six Nation's rugby coverage.
Recent jobs include The Skins series three promo for Neil Gorringe at 4 creative and the More 4 Kubrik season promo for Siri Bunford, also at 4 creative.
He starts his new job on 1 March.
Smoke and Mirrors is currently embarking on a £5m expansion programme that will see the facility more than double in size.