The agency went up against Nelson Bostock Communications, GCI and Kazoo in a competitive pitch to win the UK-focused account, believed to be worth six figures annually.
Lexis consumer director Fiona Jolly is to lead the account, reporting to Lisa Meakin, European PR manager for the search engine.
Ask.com split with incumbent Brands2Life earlier this year (PRWeek, 5 May). The firm ditched its butler icon Jeeves and relaunched as Ask.com in February.
The new site has moved away from being just a question-and-answer tool, according to Meakin.
"We have asked Lexis to initially focus on promoting the brand among consumers to drive trial of the improved service," said Meakin. The search engine has struggled recently to maintain its UK profile, with Google and Yahoo! evolving into global mega-brands.
Ask's European arm recently lost three senior executives: Adrian Cox, European executive V-P; Tony Macklin, V-P for European product management; and Rachel Johnson, European V-P of marketing.