The agencies are owned by AIM-listed Tangent Communications, which acquired Snowball in 2009 for a maximum of £3.2m in cash and shares, and were already based in the same London offices.
Tangent One managing partners Nick Gillett and Greg Jackson have left the business, although no other staff have departed.
The merger creates an agency with turnover in excess of £12m and 85 staff in London, with offices in Cheltenham, Newcastle and Australia.
The two agencies shared the Labour Party as a client, while clients from the Snowball side include Bang & Olufsen, Dunhill and Gala Coral, and those from Tangent One include Nails Inc and Carlsberg.
Bentley, who observed that generally, database marketing techniques from direct mail were not being used as effectively as they should be in digital, said clients were retreating from specialist agencies and seeking out "a new breed of full-service agency that can create powerful connections, while demonstrating effective and measurable performance".
The merger follows other below-the-line agency unions such as that between CRM agency Publicis Dialog and direct and digital agency Chemistry.
Bentley added: "Data isn't on or offline anymore – it's everywhere, and it's about being set up to harness this data and use it to drive further insight into campaigns that deliver a solid ROI. Having our own proprietary software and a large development team puts us and our clients in the driving seat."