Holborn and four fellow financial consultancies - understood to include Weber Shandwick Square Mile, CardewChancery and The Maitland Consultancy - lost out in the pitch to work for one of the world's best known sports brands.
Man United comms director Paddy Harverson said Finsbury will start work on the City and investor relations brief on 1 March.
'Holborn has done an excellent job for us over the years, but it was a question of having a fresh look at it,' he said.
'We felt the timing was right to repitch - we've had some new people join in key positions and we wanted to have a fresh look as the share price has been low and we need to work hard at the shares,' he added.
The club, which this week hit the headlines following a dressing room bust-up between David Beckham and manager Sir Alex Ferguson, recently appointed Centrica CEO Roy Gardner as non-executive chairman.
Industry speculation suggests Gardner was instrumental in the appointment of Finsbury, which has worked for FTSE 100 firm Centrica for a number of years.
Finsbury director James Murgatroyd leads the brief, reporting to Harverson and finance director Nick Humby.
Murgatroyd said: 'The focus is going to be on not only press work but also analysts, and we're doing regulatory advice where necessary.'