Mcgarrybowen has won Mothercare's creative and strategic business after a competitive pitch.
The Dentsu Aegis Network shop is tasked with revitalising the Mothercare brand to reinforce its relevance and role as the specialist retailer for new parents.
Mothercare previously worked with St Luke's on a project basis. It hired the agency in 2015 for brand repositioning consultancy, as well as an above-the-line campaign called "Welcome to the club" that launched in November last year in an attempt to introduce Mothercare to a new generation of mothers.
Mcgarrybowen will report to head of marketing Jayne O’Keeffe and chief executive Mark Newton-Jones, who called the agency "the ideal partners to introduce Mothercare to the next generation of parents".
Mothercare has been struggling in the UK; it is in the process of closing about 50 of its 137 stores by June 2019 to put itself on a sounder financial footing.
It has not previously been reported that O’Keeffe, a former Unilever and Diageo marketer who has run her own consultancy, is working for Mothercare.
"Mothercare is a rite of passage into parenthood," Jason Gonsalves, chief executive of Mcgarrybowen, said.
In May, Mothercare's then chief executive David Wood, a former European marketing director at Kraft and chief marketing officer at Tesco UK, told investors that Mothercare needed "a greater focus on value for money and to communicate this effectively to our parenting communities".
Wood had taken over the top job after Newton-Jones was sacked by chairman Alan Parker, but soon after Newton-Jones returned as chief executive when Parker was replaced by a new chairman. Wood moved to the role of group managing director.