The agency's first work will promote the store's new toddler clothing range.
Mothercare renewed its focus on marketing at the end of last year when it created a senior marketing role to target consumers more effectively.
St Luke's picked up the £3 million advertising account in February in a pitch against Miles Calcraft Briginshaw Duffy, Rapier, Mother and TBWA\London.
St Luke's is developing a customer communications strategy, which will include a television campaign.
The first work is expected to break later in the autumn.
Mothercare has overhauled its 232 UK stores, resulting in a 3.5 per cent rise in group sales to £446.9 million.
Paul O'Donnell, the chairman of OgilvyOne, commented: "We are really looking forward to working with Mothercare, especially at this interesting time in the company's development."