Campaign revealed in January that WPP had won Walgreen Boots Alliance’s $600m (£468m) global business. WPP said it would create Team W, a bespoke team that would work on advertising, media and PR across London, Chicago and New York.
As part of Team W, MediaCom UK will be responsible for helping Boots drive its retail presence in the UK and the Republic of Ireland.
Ogilvy was awarded Boots' UK creative account in February.
The retailer spent £66.8m on media in the UK last year as part of its marketing activity for Boots stores, and the Boots No 7 cosmetics and skincare ranges.
Just under half of its spend went on TV advertising last year (£32m), while 15% went towards press ads, 12% on direct mail, 8% on radio and 6% on outdoor.
Boots had used Omnicom's OMD UK for media in the UK since 2008, when it ended a four-year relationship with MediaCom. Walgreens separately appointed OMD in the US in 2013 – a year before the chain merged with Boots. Independent shop Mother has been the UK creative agency for Boots since 2003, while Omnicom's GSD&M is the US creative agency of record for Walgreens.
Walgreens Boots Alliance spent $598m on global advertising in its most recent financial year in 2016, according to a stock market filing, up 22% year on year.
Elizabeth Fagan, managing director of Boots UK and Republic of Ireland, said: "I would like to personally thank the team at OMD, our incumbent agency, with whom myself, and Boots have had a strong collaborative partnership over the past ten years, and we look forward to a similarly strong relationship with MediaCom in the future."