Cut the avocado in half, remove the pit, and scoop out the flesh. Transfer it to the bowl of a food processor.
Add mayonnaise, sour cream, lemon juice, and garlic. Blend until smooth.
Add the ranch seasoning mix and pulse a few times to combine.
Slowly pour in the milk, starting with 3 tablespoons, while blending until smooth. If a thinner consistency is desired, add more milk, 1 tablespoon at a time, until it reaches your preferred texture.
Transfer to a jar or bowl. Cover and refrigerate for at least 30 minutes to allow the flavors to develop.