Kern was hired as the showrunner when the presentation pilot became a series and shared the job of executive producer of Charmed with the show's creator Constance M. Burge for the first two seasons, when Burge departed due to disagreements.
At the end of the series, he and Holly Marie Combs took ownership of the "Hero" version of the Book of Shadows; the prop versions were sold at auctions. The "Hero" version was the main and most complete one; it was pivotal to shooting the series and was usually kept under lock and key. Kern and Holly Marie Combs currently share custody of the book, and agreed to alternate possession between the two of them, each getting the book for an entire year at a time. Despite this agreement, Kern has maintained possession since the ending of the show in 2006.