President Carter yesterday named Marvin S. Cohen to succeed Alfred E. Kahn as chairman of the Civil Aeronautics Board. Kahn resigned to become the president's top counselor on inflation policy.

Cohen, a 47-year-old lawyer from Tucson, is the newest member of the CAB, having been sworn in early this month. "It's kind of stunning to come here as a member of an agency and two weeks later find yourself chairman," he said yesterday.

Cohen was hand-picked for his new post by Kahn just as the President signed into law a bill that will deregulate the airline industry and phase out the agency's functions over the next seven years.

In an unusual letter distributed to CAB members and employees yesterday, President Carter announced his decision to name Cohen as Kahn's successor and told them, "I want to explain to you also that Chairman Kahn leaves you only with the greatest reluctance, and only because I persuaded him that I need his services worse than you do."

Cohen's views are in keeping with Kahn's; he noted that he had come to Washington committed to deregulation. "I came back here believing in these things, or I wouldn't have left my law practice," he said yesterday.

Cohen was a lawyer with the Tucson firm of Biby, Shoenhair, Warnock & Dolph from 1963 until he accepted the CAB post. Although he had no background in airline regulation, Cohen had extensive experience in utility regulation, both as an attorney for regulated industrial clients and on behalf of those doing the regulating in his home state of Arizona.

Cohen's arrival at the board had given it a full complement for the first time since June; now President Carter will have a fifth slot to fill with Kahn's departure. Three of the remaining four members have been Carter appointees.