Yellow mustard – the most common variety in America – is a simple mixture of ground mustard seeds, turmeric, salt, vinegar, and paprika, which means it’s vegan. No matter what brand of yellow mustard you buy, the basic formula is likely to be the same. That being said, other varieties of mustard might not be vegan. For example, honey mustard is definitely not vegan. Some dijon mustards may not be vegan in the strictest sense because they contain white wine, and cheap whites have a tendency to not be vegan – at least based on what I’ve looked up on Barnivore.
To play it safe, if the mustard in question is anything other than plain yellow mustard, take a look at the ingredients label to find out what else is in it. Some labels might list “natural flavors.” If that’s the case, there’s a good chance those flavors are vegan, but there’s no way to know for sure unless you contact the manufacturer and they are able to tell you.