Iremonger replaces Stewart Atkins, who is moving over to become the director of digital innovation and technology. The two will work together to take Proximity Digital into its next phase of growth.
Iremonger's career spans 17 years across the communications and digital sectors. He led Unit9 for three years, during which time the agency grew to 15 staff, with clients including The Sunday Times, Vodafone TV and Sony PlayStation.
He joined Unit9 from Sleeper, a joint venture between the film production company Blink and the digital agency Deepend, which he set up to bridge the skills of the two companies. Before that, Iremonger worked at the brand experience agency Maritz Communications as its head of film and video.
His early career included four years at the marketing services company Interlink and a spell at Diverse Image in Dublin.
The Proximity chief executive, Chris Thomas, said: "Mark is, without a doubt, one of the most creatively led managers on the digital scene in the UK. He will provide the perfect complement to our strong existing digital team and we're thrilled he's joining us."