The move reunites Patten with Proximity's chief executive, Chris Thomas, following their time together more than 20 years ago at Abbott Mead Vickers BBDO.
Patten, who is currently the managing partner at Heresy, is Thomas' first senior hiring since he joined Proximity in the summer. It will boost the senior management line-up following the departure of the co-founders Simon Hall and Chris Barraclough earlier this year.
Thomas said: "Tim's appointment is very exciting. He brings to our management team strong client experience, a uniquely integrated background and a refreshingly entrepreneurial spirit. He sets high standards for himself and all who work with him."
Patten's brief, when he arrives in January, will be to spearhead the Royal Mail account and to strive for better client integration across the AMV Group.
Patten, who has worked with high-profile clients including Heinz, Guinness and Vodafone, began at DMB&B before moving to AMV.
From there, he relocated to the client side, gaining experience at BT's marketing communications division, where he worked on the "it's good to talk" campaign.
Three years later, he helped launch BT's internet and multimedia group, before leaving to set up Harrison Patten Troughton Brand Response.
At HHCL & Partners Group, he established the digital and direct agency Heresy.