The maternity, baby and childcare retailers appointed The Search Works after a competitive pitch.
The win will see The Search Works focus on driving sales online as well as on the high street, by raising the awareness of both brands in search listings.
Leighanne Miles, head of group e-Commerce at Mothercare and ELC, said the companies' online sales channel was growing at "a tremendous rate and it has become clear that we need to work with an agency with the prerequisite retail expertise, scale and strategic consultancy".
The Search Works' appointment follows the acquisition of ELC by Mothercare last year, which prompted a full review of all search marketing.
The two retailers still operate as separate outlets, but share a marketing strategy.
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