Cass, who has been running Carat's global Procter & Gamble account, joined Carat in 1995.
He first served in the UK office as client director of Reckitt & Colman, before becoming the youngest board director at Carat UK in 1998.
In 1999, Cass worked as communication planning director of Carat UK and in 2004 joined the New York office after leading the pitch team that won a share of P&G's strategic planning business.
Cass said: "My number one job is to make sure we have stability and make sure customers are happy and comfortable as we move further into a very uncertain economic period.
"I want to get my feet under the table and understand all of the customers we service and get some level of perspective on all of this and make sure we are delivering a high level of service to our clients in the most efficient and economic manner."
A replacement for Cass has not yet been announced.
Sarah Fay, chief executive of Aegis Media North America, said Sorokin's departure from the agency was "mutual".
Sorokin leaves Carat after two years at the company. He joined as president of Carat Fusion in 2006 and became president of Carat when the agency merged with Carat USA in July 2007.
He said: "I am truly proud of what I have accomplished and have tremendous respect for the team at Carat. But at this point in time, I am considering other opportunities focusing on digital marketing innovation."
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