The WPP agency pitched for the business in a review handled by Oystercatchers. Fidelity previously handled media in-house.
Fidelity is planning to "substantially grow" its business, and increase awareness of the retirement solutions it provides.
MEC has been tasked with helping the brand "take its media focus beyond press and out-of-home" with a data-led approach to media planning and buying. Grey London is the brand’s creative agency.
James Harris, head of marketing for UK financial services at Fidelity, said: "With changing client behaviours and the growth of digital, the need to be insight driven and optimise content across communication channels is paramount, so we wanted a partner with a strong strategic platform and a good understanding of our brand, end consumers, and financial advisers."