Macnamara replaces Tim Williams, who was hired in January 2010 from Utarget.
In his new role, Macnamara will head up the company's sales division, focusing on growing existing client relationships and business growth, through the strategic promotion of Tremor Video's media and technology solutions across the UK.
He will report to Dan Ruch, vice-president for Europe at Tremor Video.
Macnamara was agency group head at Yahoo for nearly three years until he left the company in July.
He was previously head of advertising at RealNetworks for nearly four years. and agency sales manager at AOL for more than four years.
Tremor Video, which recently rebranded from Tremor Media, sells in-stream ad products, and claims to reach more than 3,000 publishers who service in-stream video.