The Interpublic agency won the brief in partnership with the digital agency Essence Media, which picked up the online portion of the business, after a pitch against undisclosed agencies. The incumbent was Brilliant Media.
The site, which maps an individual's family tree, launched a campaign with the tagline "Find us" last year through the incumbent creative agency, HMDG.
Jonathan Saverimuttu, the Ancestry.co.uk acquisition marketing director, said: "The appointment of Initiative and Essence Media is a key part of driving Ancestry.com growth in the UK market."
Gary Birtles, the chief executive of Initiative UK, said: "Ancestry has a great product and we are delighted to be working with their UK business as they seek to grow their audience."
Ancestry.co.uk has the world's largest collection of family trees, with more than 1.7 billion profiles from more than 100 countries.
The site faces competition in the UK from smaller sites including Family Search, but also from the increasingly popular Genes Reunited, an offshoot of Friends Reunited.