The group was bought from publisher Trinity Mirror by Iliffe News & Media's division LSN Media in November. Iliffe initially retained Trinity Mirror's sales house Amra for the publications' national sales.
Mike Richardson, managing director of LSN Media, said Clacksons was awarded the contract because it represented LSN Media's interests across Bedfordshire and Buckinghamshire, and has made "great progress" developing the national business for its Milton Keynes title, MK News.
The Northamptonshire Herald & Post Group's two editions publish more than 75,000 copies a week across Northampton, Towcester, Brackley, Rushden, and Welling- borough.
Adrian Worsley, managing director of Clacksons, said the titles covered "highly desirable markets" and complemented geographic regions to Clacksons' existing portfolio.
Clacksons handles the national sales for 122 newspapers, 60 magazines and 187 websites, including the titles of regional publisher Archant.