The appointment, which was made without a pitch, lands Mother a position on the J&J roster, where it joins DDB London, Lowe London and McCann Erickson. Media will be handled by Initiative.
The Mother partner Andy Medd said: "Working on such a significant product launch gives us an opportunity to shake up what has become a relatively dull market."
The J&J franchise director for Europe, the Middle East and Africa, Ryan St George, said: "Mother has consistently impressed us with its calibre of thinking, creative insight and sheer passion and enthusiasm for our business."
J&J bought the Rembrandt range of whitening products from Gillette for an undisclosed sum in October 2005.
The European push comes as J&J's rival Colgate-Palmolive moves to increase its UK market share with the £3 million launch of Colgate Total Professional Weekly Clean - a premium dental cleaning product.