Robert E. Allen, a vice president of American Telephone & Telegraph Co., will become president of the Chesapeake and Potomac Telephone Companies, effective Sept. 1.

Allen, AT&T's 46-year-old vice president for business services, will succeed Samuel E. Bonsack, 62, who is retiring after 41 years with Bell. Bonsack spent his last seven years as president of the four C&P companies.

Allen, a native of Joplin, Mo., was graduated from Wabash College of Crawfordsville, Ind., and joined Indiana Bell as a traffic student in 1957. He held a variety of traffic and commercial department assignments in Indiana until 1972, when he was elected vice president for rates and revenues and secretary-treasurer of the Indiana company.

In 1974, Allen moved to Philadelphia as vice president and general manager of the eastern area of Ma Bellhs Pennsylvania unit. Two years later, he was named vice president for operations and a director of Illinois Bell in Chicago. He was appointed to his current position with the parent company on Aug. 1, 1978.

Allen is a director of City Federal Savings and Loan of New Jersey and of Pacific Northwest Bell Telephone Co. in Seattle. He is also a trustee of Wabash College.

Bonsack will complete a career with Bell that began in Richmond in 1940, following his graudation from Virginia Polytechnic Institute with a degree in business administration. He held a variety of assignments throughout Virginia and the district before being named general traffic supervisor in Washington in 1952. A similar job in Richmond followed, and Bonsack moved to Charleston, W. Va., as general traffic manager in 1958. In 1959, Bonsack moved to the comptrollers department of AT&T for two years, returning to Richmond three years later as vice president for Virginia.

Bosnack moved to New York in 1970 after being named vice president for market services and plans of AT&T. On Aug. 15, 1974 he was elected president of the C&P companies.