Retired Navy Capt. Detlow M. Marthinson Jr., 45, whose last assignment was with the office of the commander, Atlantic Fleet, Norfolk, died of cancer Monday at the Portsmouth, Va. Naval Hospital.

He had retired in August after serving for a year as deputy assistant chief of staff for current operations.

Capt. Marthinson was born in Washington. He graduated from St. Albans School and in 1956 from Washington and Lee University. He entered the Navy that year.

He had served on destroyers as executive officer and as commander. In 1968, he was in Vietnam and was wounded in action.

From 1969 to 1972, Capt. Marthinson worked in connection with advanced ship types and combatant crafts at the Navy Department here. During that period, he and his family lived in Alexandria.

His decorations included the Legion of Merit and the Bronze Star Medal.

He is survived by his wife, Sara G. Marthinson, of the home in Virginia Beach; a daughter, Sydney Elizabeth, and two sons, Christian Detlow and Matthew Cleaver, of Virginia Beach; his mother, Ruth Coleman Marthinson, and a sister, Sherrard M. Addison, both of Washington, and another sister, Jan Mewhinney, of Alexandria.

The family suggests that expressions of sympathy be in the form of contributions to the American Cancer Society.