Slater is St. Louis' director of player personnel, joining the franchise after an eight-year stint with the Dodgers as director of scouting operations, director of baseball operations and director of professional scouting.
He joined the Dodgers when Kevin Malone was general manager, having previously worked together with Baltimore.
"For me to walk back into this stadium, I really have mixed feelings," Slater said Sunday as the Cardinals worked out at Dodger Stadium. "I have a lot of good friends here."
Slater said he credits Cardinals ownership for "creating an environment of stability" that translates into the success the organization has had in recent years.
Ken Gurnick is a reporter for MLB.com. Chris Haft is a reporter for MLB.com. Read his blog, Haft-Baked Ideas, and follow him on Twitter at @sfgiantsbeat. Austin Laymance is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.