The agency has snatched the account, worth an estimated £4 million, from EHS Brann, which had won the business last December.
Claydon Heeley will handle all direct marketing for The Guardian, The Observer and Guardian Unlimited.
Last year Claydon Heeley was appointed to develop sales promotion for Guardian Newspapers, but the newspaper group has decided to consolidate the account.
It was originally won by Brann before its merger with EHS. The win followed a pitch against the previous incumbent, KLP Euro RSCG, and several other agencies.
Marc Sands, the marketing director at Guardian Newspapers, said: "Brann pitched at the end of last year as Brann and then it merged with EHS, and it took time for them to work out how this would work.
"You buy into companies and key individuals and a couple left and they were more important to me than the company. I was satisfied with their work but there is a real cultural fit with Claydon Heeley."
Claydon Heeley's previous work for The Guardian has included promotional activity for its World Cup coverage, which included a free set of football playing cards called "Top Chumps".
The agency recently hired Mike Welsh, the former client services director at EHS Brann, to work on Guardian Newspapers and other accounts as a senior business director. He had worked on Guardian Newspapers' account at EHS Brann.
Guardian Newspapers' creative account is held by BMP DDB. PHD handles media.