The Chime Communications team, which combines people drawn from both agencies, has won a seven-month contract with the option to extend for a further year.
The work is being overseen by the National Lottery Promotions Unit (NLPU).
Fifty agencies are understood to have applied for the work, which was reduced to a shortlist of ten, and then a final shortlist of three.
The purpose of the brief is to raise awareness of the £35m available for good causes among the UK public, and will have a strong digital focus.
NLPU director Vicki Kennedy said: "National Lottery players raise a staggering £35m for good causes all across the UK. No matter what your interest you’ll be benefitting from this funding but the likelihood is you won’t even realise it. The Chime team have some great creative concepts for getting this message out to consumers and we are really looking forward to working with them."
The NLPU is a joint venture between Camelot, Lottery Distributors and the Department for Culture, Media and Sport and was set up in 2002.