The agency was chosen by Allied Bakeries after a four-way pitch to handle consumer, trade and regional PR.
Lexis takes over the retained account from York-based incumbent Three60, which held the contract for more than three years.
Lexis will launch and promote new products under the Kingsmill brand in early 2011. Kingsmill is the firm's biggest selling brand, with 15 different products on supermarket shelves.
The agency will be launching a national and regional consumer and trade PR campaign to raise awareness of all of the brands and support individual bakeries across eight UK regions.