The companies will combine under the name Carat Interactive and have approximately 100 employees with locations in Boston, Los Angeles, New York and San Francisco. Clients include Adidas, AT&T Broadband, Bank of America, Autodesk and Pfizer.
The terms of the acquisition were not disclosed.
"We have always believed, and continue to believe, that digital technologies will transform the media marketplace," David Verklin, CEO of Carat North America, said in a statement.
As a result of the acquisition, Sarah Fay will retain her duties as president of Carat Interactive. Kate Everett-Thorp, CEO of Lot21, will become chairman and Eric Wheeler, president and chief operating officer of Lot21, will act as COO.