The agency won the business after a competitive pitch against We Are Social, Blast Radius and Frank, with the incumbent, Skive, pulling out earlier in the review.
JWT has handled the Kit Kat advertising account since the chocolate brand's creation. The agency invented the strapline "Have a break. Have a Kit Kat", with the first-ever ad to feature the famous line appearing in 1957.
The new assignment means JWT will also be responsible for digital activity including Kit Kat's website, online advertising and social media execution, including community management across Kit Kat's social media platforms such as Facebook.
James Whitehead, an executive partner at JWT London, said: "Kit Kat is a great prize - it's already among the top ten most-engaged brands on Facebook and we will be helping Nestlé achieve their ambition to turn this much-loved brand into a world-class operator digitally."
Kit Kat is the fourth-largest UK chocolate brand with estimated sales of £120 million in 2011, according to Mintel.Follow @Jezzalee