With the retirement of Tony LaRussa, Andy Reid has now become the fourth longest tenured coach amongst the major American pro sports leagues. Here's the list of the coaches ahead of him and when they had their first year.
Gregg Popovich, San Antonio Spurs - 96-97
Lindy Ruff, Buffalo Sabres - 97-98
Barry Trotz, Nashville Predators - 98-99
Andy Reid, Philadelphia Eagles - 1999
So obviously he's also the longest tenured coach in the NFL, which he became after Jeff Fisher left Tennessee. Bill Belichick started in New England the year after Reid came here.