The following are known Action Replay Codes for Mario Tennis on Nintendo Game Boy / Color (GB / GBC). These codes were also known as GameShark Codes and in most cases will work on both devices.

Device Compatibility: Action Replay & GameShark

P1 Quick Win
9107e2c8

P1 Never Wins
9100e2c8

P2 Quick Win
9107e3c8

P2 Never Wins
9100e3c8

Max Score Mini Games
91637cc4

Fast Stats Gain Alex and Nina
910118c9

Fast Stats Gain Harry and Kate
910158c9