In a rimmed skillet, saute bacon until crisp. Remove from skillet and reserve dripping in the pan.
Add onion and saute till tender. While onions cook, whisk together the mayo, vinegar, and sugar, and set aside.
Add green beans and bacon back to pan and remove from heat. Add the sauce to the pan. Gently toss the green beans in the sauce till they are coated and warm. Serve immediately.