Smee is based in London and reports to Daniel Cohen, who arrived at Lewis earlier this month in a senior vice president role having previously been Grayling's global head of brands.
In May 2014, Lewis acquired UK-based digital marketing agency Purestone in an all-cash transaction, with its 20 staff and founding duo of Butler and Smee all moving over.
The Lewis Purestone business unit will now be led by by the firm's creative director Simon Billington.
Lewis has a 60-strong EMEA digital marketing practice with half the staff based in London, and the rest across the agency's other European offices, led by Cohen.
Smee said: "Over the last three years with Lewis Purestone, I have seen the increasing demand for more integrated services. There's a very real opportunity to drive our digital marketing practice across EMEA."
Of Butler's departure, Cohen said: "After more than a decade building the agency and following the successful integration into Lewis, Simon decided to explore a new career opportunity. We are grateful for everything Simon did and achieved with Lewis Purestone, and wish him well for the future."