Paul E. Reichardt will retire next month as board chairman of Washington Gas Light Co., ending a 43-year career, the utility announced yesterday.

Reichardt, 64, will be succeeded Feb. 1 by Donald J. Heim, 55. Heim has been the company's president since 1977 and was named chief executive officer a year ago. He will retain both positions.

In addition, Washington Gas announced that Norma Maine Loeser, 60, had been elected to its board of directors, becoming the first woman on the nine-member board. Loeser is dean of the George Washington University School of Government and Business Administration.

A spokesman attributed Reichardt's departure to the company's mandatory retirement age of 65. Reichardt will be 65 on April 26. Loeser will fill the board vacancy caused by Reichardt's retirement.

Reichardt, a chemical engineer, had been chairman since 1974. Hired as a cadet engineer in 1940, Reichardt held positions during the next 22 years as a chief chemist and in distribution, appliance service, engineering and development, value analysis and general administration.

He became special assistant to the president in 1962, vice president for corporate planning and services in 1964 and president in 1969.

He was named chief executive officer four years later. In 1975 and 1976, he was board chairman of the American Gas Association. Heim, a mechanical engineer, joined Washington Gas as a cadet engineer in 1950. During the next 14 years, he held positions in distribution, operations, value analysis, appliance service, marketing and finance. He later was promoted to assistant to the president and director of corporate planning, and in 1973 he was named vice president for corporate planning and services.

Heim is an executive committee member of the Associated Gas Distributors and a director of the Greater Washington Board of Trade, American Gas Association and Maryland Chamber of Commerce.

Loeser is a retired Air Force lieutenant colonel and former managing director of the Civil Aeronautics Board. She has been dean at George Washington since 1978.