The duo will quit Fallon's New York office to link up with Andrew Deitchman, who joined from WPP's Red Cell in June. All three will be installed as partners in the New York business, which is expected to open some time in the next year.
The Mother partner Mark Waites called the pair an "outstanding creative team". He said: "We have talked to so many people about starting the New York office, but they had that extra something. We needed people we could trust 100 per cent with our brand and we feel they can go away and simply get on with it."
Karlsson, an art director, and Malmstrom, a copywriter, worked for the Stockholm-based agency Paradiset DDB before joining Fallon Minneapolis in 1996.
While at Paradiset, they were best known for their work for the Diesel brand. Since then, they have worked with the directing collective Traktor to create idents for MTV's hillbilly quartet, the Jukka Brothers, which won acclaim at Cannes in 2000.
Other credits on their reel include Lee Jeans, Virgin Mobile and Miller Lite.
The pair, who relocated to Fallon New York last year, said in a joint statement: "Joining Mother is obvious to us. The world needs Mother right now. It's a refreshing, brave and global little company in a very boring and consolidated industry. We are very happy to be able to contribute to the success of a company like that."
Mother was launched in London six years ago by Waites and the other founding partners Stef Calcraft, Robert Saville and Andrew Medd.