The appointment comes as Saga increases its marketing activity in a bid to build a more dynamic brand and broaden its customer base.
Saga, which provides package holidays, insurance and personal finance, previously worked with the integrated agency DCH on parts of its business.
McCann's first work for Saga launches later this month and consists of a series of three television spots promoting insurance, cruises and holidays.
Earlier this month, Acromas Holdings, the parent of the Saga and AA brands, appointed SapientNitro to redesign and build the websites for both companies.
Alan Griffiths, the Saga Holidays marketing director, predicts the new campaign will mark a turning point for the brand "despite extreme winter conditions and a tough, tough economic climate".