Lonergan's role will be to manage the day-to-day operations of the agency and grow its client portfolio across a wider range of categories.
Its existing clients include Unilever ice cream brand Ben and Jerry's and Pernod Ricard's Beefeater Gin.
Lonergan has worked client side at Unilever in the past but has more recently worked in client handling roles for digital agencies. Prior to joining Sapient in 2007 she worked at 20:20 London and eMC Saatchi.
Kevin Malone, the founding partner of Maynard Malone, said: "I'm delighted that Maggie Lonergan has come on board, bringing with her a wealth of digital and communications experience, but also embodying the values and vision of Maynard Malone, namely to be recognised as the agency brands most trust with their customer relationships."