Byrnes had shown interest in joining the San Francisco Giants after he was designated for assignment by the Diamondbacks, however, on January 29, 2010, Byrnes signed a one-year, $400,000 contract with the Seattle Mariners. To make room for Byrnes on the 40-man roster, the Mariners designated first baseman Tommy Everidge for assignment. The Diamondbacks are still responsible for the remaining $11 million on his contract he had with them before being released. Byrnes was slated to share playing time in the outfield with Milton Bradley at the time of his signing. Byrnes said this about being signed by the Mariners: