Mariano (Joe) Cortez, 46, an Agriculture Department employe for more than 20 years who had lived here since 1969, died in the Metro subway derailment Wednesday. He lived in Riverdale.

Mr. Cortez joined the Agriculture Department's packers and stockyards branch in Los Angeles as an accountant trainee in 1959. He came to Washington as an auditor with the P&S Reports Unit. Since 1972, he had been an automated data processing system specialist in the packers and stockyards branch.

He received a certificate of appreciation in 1966 and a certificate of merit in 1974, both from the Department of Agriculture.

Mr. Cortez was a native of Brooklyn, N.Y. He served in the Army from 1953 to 1956 and was a noncommissoned officer in the 82nd Airborne division. He was a 1958 graduate of Woodbury University in Los Angeles where he earned a bachelor's degree in business administration.

He earned a certified public accountant's certificate in 1972 and was a member of the American Institute and the Maryland Association of certified public accountants.

Survivors include his mother, Mrs. Ramona Suarez of Queens, N.Y., a sister, Mary Morin, a half-sister, Diana Suarez, and a half-brother, Richard Suarez, all of Brooklyn.