Mikkelsen was a durable sinkerballer who filled various relief roles for several ML clubs. As a Yankee rookie in the 1964 World Series, he was the victim of Cardinal Tim McCarver‘s game-winning three-run homer in the tenth inning of Game Five. At his best, he appeared in 71 games with nine wins and 14 saves for the 1966 Pirates. He finished with 49 career saves.