Sean Landeta: 21 seasons
Landeta could’ve been higher on this list, but his first two full seasons in professional football were played in the USFL for the Philadelphia/Baltimore Stars. He joined the Giants in 1985, winning two Super Bowls before going onto play for the Rams, Bucs, Packers and Eagles. He was the last active NFL player to have appeared in the USFL and announced his retirement on March 6, 2008, which was the 25th anniversary of his first game in the USFL.