Lewis has given his notice to the board and said he wants a smooth transition to his successor, though no exit date has been agreed.
The FTSE 250-listed company, which helps consumers switch contracts for better deals for their energy, telecoms and other bills, noted that Lewis has got the switching bug and wishes to “pursue his career in a new direction”.
A formal search has begun for a new CEO, including internal and external candidates.
After taking over from Peter Plumb in April 2017, shares in Moneysupermarket rose to an all-time high by July last year before tumbling in the second half of the year as growth appeared to slow.
Third-quarter revenue growth creaked to 4%, compared to the 15% and 19% growth seen in the preceding quarters.
Analysts said that the group's weaker performance was due to subdued market conditions rather than strategy, as the company is in the second year of Lewis’s so-called ‘Reinvent’ strategy, which aims to personalise deals for customers, as well as offer new products such as mortgages.
With the CEO announcement coming a day ahead of full-year results, the group said its performance was in line with current market expectations for 2019.
Lewis's departure follows a year of significant changes at the top of FTSE 350 companies, with around a fifth of FTSE 100 company bosses leaving and dozens of those in the FTSE 250 too, with the trend continuing in 2020.