Joshua was awarded the UK business as part of a renewed focus on integration.
Potential conflict issues with Grey London's Masterfoods account were also cited as a reason for the move.
Joshua has been tasked with creating an advertising idea for the UK market, which will work across all media. The campaign will break later this spring.
"We are delighted at this news," Michael Miley, a board director at Joshua, said. "It confirms our ability to work across the line in all media."
The last work Grey London created for Ambi Pur in the UK ran in November.
The £1.8 million campaign featured an animated character called the Smell-Catcher, which flitted around gathering smells to take to the Ambi Pur factory for use in the company's products.
The campaign promoted the company's winter range of plug-ins and scented candles. Ambi Pur leads the plug-in market with a 42 per cent share. It has a 4 per cent share of the scented-candle market.
In November, Sara Lee consolidated its £170 million pan-European media account into OMD. The account was previously split between nine agencies. In the UK, it was held by ZenithOptimedia.