The agency pitched against Beattie McGuinness Bungay, Karmarama and Euro RSCG to win the business.
Euro RSCG was appointed to handle advertising for the brand in 2007, and has been producing work on a project-by-project basis since then.
W&K will now work on all of Lactofree's national advertising campaigns, with its first ads expected to roll out early next year.
The appointment strengthens the agency's position on the Arla Foods roster. It already handles advertising for Lurpak and Cravendale.
Jess Hardcastle, the group brand manager at Arla Foods, said: "W&K has delivered strong campaigns for Lurpak and Cravendale due to their ability to produce creative work that stands out from the crowd. We are confident their campaigns for Lactofree will be as successful."
Arla Foods is also in the process of reviewing its pan-European branding arrangements. W&K has been knocked out of the contest, with Saatchi & Saatchi and BBDO battling it out in a final procurement phase.