Leo Dewey Welch, 80, the first chairman and chief executive officer of Communications Satellite Corp. (Comsat), was killed in a collision Saturday of an automobile and an oil truck near Cuernavaca, Mexico.

Three other persons in the car with Mr. Welch also died in the crash. He was vacationing in Mexico at the time.

A well known industrialist and banker, Mr. Welch wa chairman of Standard Oil Co. of New Jersey when he was appointed by President John F. Kennedy to head the newly formed Comsat in early 1963.

Comsat was established as a private enterprise under a franchise from Congress to operate this country's part in a global satellite system for television, radio and telephone. Mr. Welch handled the financial aspects of the operations, including the sale of stock.

He retired from active direction of the corporation in 1965, explaining that he wanted to slow down his pace and nothing that the "establishment phase of the corporation" had been completed. He remained on the board of directors until 1977.

Mr. Welch was born in Rochester, N.Y. He served in the Navy Reserve in World War 1, and graduated from the University of Rochester. He them became a trainee with the First National City Bank of New York and served in its branches in Argentina, Chile and Uruguay.

He joined Standard Oil of New Jersey (now Exxon) as treasurer in 1944, became a director in 1953, and was elected chairman of the board in 1960. He retired from that post in 1963. His work took him on extensive travel. Added to his earlier career as a banker in South America, this international experience made him the ideal choice for globe-trotting chief of Comsat, officials said.

From 1971 to 1976, Mr. Welch was chairman of the National Arbitration Panel. He lived in Syosset, N.Y., and frequently dropped in at Exxon offices in New York City until his death.

He is survived by a daughter, Mrs. Emmet Whitlock, of Oyster Bay, N.Y., and two grandchildren.