Mayonnaise in stand mixer is very runny I've frequently made homemade mayonnaise in my food processor following Alton Brown's recipe, and consistently had good results.

Last week the food processor was busy with something else, and so I decided to make the mayo in the stand mixer. Using the whisk attachment and turning it up to high speed, I got everything emulsified but the resulting mayonnaise was very runny -- not broken or separated, but much less viscous than what I get from the food processor. It's been in the fridge for about a week, and is still the same consistency (emulsified but runny).

I assume it got too much air whipped in or something. Is there a way to use a stand mixer for mayonnaise, or does it really need the speed of a food processor or blender?
I'm taking a stab here, but since you're accustomed to making mayonnaise in a food processor, you probably already have a habit for how you pour in the oil. Since a food processor moves so fast, it can emulsify quickly, so you can pour the oil in relatively quickly. 

A mixer is much slower, like using a whisk, you need to pour the oil very slowly, so every drop becomes completely emulsified before more is added. So try it slower next time.

For this mayonnaise, try to fix it as if it's broken, as in this video

Put another egg yolk in a mixing bowl and slowly add your thin mayo while whisking (or mixing in your mixer). Then slowly add more oil until the consistency is right.

BTW: See how slowly she's pouring?