Ten Alps has made an initial payment of £1.65m and will make a possible further payment of up to £1.45m if Mongoose achieves earnings before interest and taxation of £725,000 in the first 12 months.
An additional £300,000 may also be paid according to performance.
Mongoose sells advertising across specialist print and digital media on behalf of clients including Newsweek, National Geographic, Time Out and the National Childbirth Trust.
The London-based company currently has 19 staff and reported gross revenues of £5.2m for the year ending December 27 2006.
Mongoose will be run as an autonomous business within the communications division of Ten Alps.
Two weeks ago Ten Alps bought corporate social responsibility arm specialist DBDA for £3.3m.
This is Ten Alps' 14th acquisition since listing on the Alternative Investment Market in July 2001. The company also owns factual independent producers Brook Lapping, Blakeway/3BM and Hart Davies.
Brook Lapping is a 70% shareholder in the consortium behind the channel Teachers TV.