1. Someone who is able to converse easily and pleasantly with other people in a social setting or event. I was worried about bringing my new girlfriend out to a party with so many people he had never met before, but luckily she's a really good mixer.

2. A non-alcoholic beverage that pairs well with a particular type of alcohol. I turned my nose up when he started pouring tomato juice into the glass, but apparently it's a really good mixer with vodka.