In the episode Success Perm, Mitch was told to fill up the restaurant with as many people as he could. One of them was the girl he was dating, Charlene. Charlene was not originally part of the plan. Mitch invited Charlene over for their date and it just so happened to be at the right time. When Mitch was on his date with Charlene, she told him that she felt like she could really open up to him. Their date was then interrupted by a phone call from someone booking reservations.
In the episode License to Sell, it can be implied that their relationship is over, because Mitch mentioned that he was sexually intimate with Nancy. However, considering that he was cheating on his wife when dating her in the first place, it's also just as likely that he was cheating on her too.