Cordero, who is looking to make a comeback next year, is in great shape after losing 25 pounds by being on Weight Watchers.
Cordero was arguably the first star in Nationals history, saving 47 games with a 1.82 ERA in 2005, the team's inaugural season.
He pitched for Washington until 2008, when a torn labrum in April knocked him out for the rest of the season. After being released by the Nationals at the end of that season, he signed with the Mariners, making nine appearances in 2010, his last big league action.
Cordero is expected to make an appearance at Nationals Park on July 20 and throw out the first pitch before the Nationals play the Braves.