The group could potentially pay out a further £9.45m in 2010 if April-Six meets "challenging" performance criteria based on its pre-tax profit performance over the next three years.
April-Six reported £3.05m turnover last year from consultancy, marketing and communications services to software, reseller and telecoms companies in the UK and abroad.
The Middlesex-based company was set up in 2000 by founding partners Steve Crowther and Fiona Shepherd and currently employs 54 staff, who will now join Mission's payroll.
Iain Ferguson, Mission Marketing's chief executive, said: "IT is another one of the sector winners we have identified which have super strong fundamentals, significant potential for growth and offer scope for layering new services."
Fiona Shepherd, managing partner of April-Six, said: "Joining the Mission provides us with an ideal partner to help us develop our business even further, and to offer additional services to those of our clients who have other specialist needs."
Earlier this week, Mission Marketing acquired specialist property and real estate marketing agency for Bastin Day Westley for £6m with a three-year maximum earn-out of a further £7.8m.
The agency counts housing companies Bellway and Crest Nicholson among its clients and has offices in Colchester, Essex and central London.
Mission is chaired by Conservative Party chairman Francis Maude.