Lewis, who held the position of head of agency sales since joining in August 2001, is leaving the company to become head of agency sales at rival Yahoo!.
Townsend has been at AOL for 15 months, where he worked as joint group head with Cluny Venables in a 36-strong team. Prior to joining AOL, Townsend worked for Classic FM and Clear Channel.
Giles Ivey, sales director at AOL, said: "Luke offers us a strong multimedia background for FMCG. He understands multimedia campaigns very well, having worked across radio, online and outdoor campaigns.
"Luke is someone who genuinely cares about advertisers and their needs. He leads from the front and offers compelling solutions to creating effective ad campaigns. He has a strong knowledge of online and can complement that with his background in broadcast and outdoor advertising."
Ivey explained that agencies were becoming more involved in the development of online advertising "especially with more traditional businesses like Sainsbury's and Tesco, which are increasingly making online a central part of media planning."
Last month, AOL UK announced plans to create an integrated social networking site to rival Bebo and MySpace.
Ivey said: "[The plans are] definitely something for the future, but it's our intention to develop this venture throughout 2007. Discussions are still very much in the pipeline at this stage."
Time Warner, which owns AOL UK, is currently reviewing all of its European business, and is reported to have been in discussions with BSkyB about the sale of AOL UK's internet access business.
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