He joined MS&L late last year to head the agency's London corporate practice (PRWeek, 23 November 2009). He was previously a director at Cohn & Wolfe.
Maguire, who has left MS&L 'by mutual consent', plans to set up his own corporate PR shop.
MS&L CEO Kelly Walsh declined to comment on the circumstances surrounding Maguire's departure, but said the agency was now seeking his replacement.
The corporate team, whose clients include Coca-Cola, P&G, Atos Origin and Corel, will be headed by director Vicki Newbold until Maguire's replacement has been found.
Maguire has more than 20 years' experience in communications, having worked at agencies including Bell Pottinger and Launch Group, and in-house at the Granada Group.
During his time at Cohn & Wolfe, Maguire worked across a range of corporate and public sector clients.