McIlvaine, 64, has most recently been an assistant to the GM with the Twins since 2000, working with scouting at the Major League, Minor League, amateur and international levels.
McIlvaine is the third special assistant that Zduriencik has hired in recent days, joining former Pirates coach and executive Pete Vuckovich and long-time Mariners coach Roger Hansen, who was given a promotion.
Every Major League team has several special assistants to the general manager. The Mariners already have Tony Blengino, John Boles and Ken Madeja in those roles as well as senior advisor Ted Simmons. The new hires are adding to that group, though Boles is leaving at the end of this season after seven years in the organization.
McIlvaine spent the last 12 seasons with the Twins. He was the Mets executive vice president of baseball operations from 1993-97 and the Padres general manager from 1990-93. Prior to that he spent a decade in the Mets' scouting department.
"Joe has had success as a general manager, a scouting director (including internationally), a front office executive and in the role of special assistant," Zduriencik said in a statement. "He is a very versatile talent evaluator, and adding him to our staff in Seattle makes us a stronger organization. I am looking forward to working closely with him."
McIlvaine was a right-handed pitcher in the Tigers' Minor League system from 1969-73 before getting into scouting.