Past Mordecai is Mordecai from an alternate timeline and the main antagonist of the season 3 finale episode "Bad Kiss" of Regular Show.
In an attempt to revert time, Mordecai and Rigby traveled in time with a time-machine to prevent Past Mordecai from eating smelly foods. However, they tried to force Past Mordecai into use a mouth wash, but when Mordecai's plan goes wrong, Past Mordecai takes the time machine from his hands and goes into the future to kiss Margaret.
Rigby had another time-machine and they go after Past Mordecai to prevent him, when travelling in time. Mordecai tricks Past Mordecai so that he can keep him from kissing Margaret. When they finally reach Past Mordecai, Rigby uses one of the time-machines to take Past Mordecai to the future again.
Then, Mordecai breaks the first time-machine in the past and Rigby breaks the second time-machine in the future, likely erasing Past Mordecai from existence as well.
Past Mordecai's personality is almost identical to his good counterpart, however, Past Mordecai is more evil and arrogant, he will do nearly anything to kiss Margaret even defeat, humiliate or even kill his good counterpart.
- Past Mordecai is defeated by Mordecai and Rigby uses the time machine by being trapped in the time warp.
- He is one of the many evil versions of the characters, like Iacedrom & Ygbir, Ybgir, Guardian of Secrets, Hologram 9000, Stress and Future Mordecai and Rigby.
- Unlike the other evil versions, Past Mordecai is identical to his good version.
- However, he has pure white feathers in his chest, unlike Mordecai in some episodes.
- He is very similar to Rimmer's Duplicate from the famous Red Dwarf episode "Me 2" due to them both being clones of protagonist villains who continuously torment and aggravate their real counterparts, both were also erased from existence at the end of their episodes.
- It's possible that one of the reasons for which Past Mordecai didn't care on killing Mordecai although that could threaten his own existence is because Mordecai came from the future while the Past Mordecai was, as his name says, from the past, therefore allowing him to end with his future self if he saw it necessary because he was the only one who mattered to preserve Mordecai's existence.