Ensure all ingredients are at room temperature for proper emulsification. If the soy milk is cold, warm it slightly by microwaving for about 10 seconds.
In a tall immersion blender beaker or jar, combine the soy milk, apple cider vinegar, garlic powder, sea salt, and Dijon mustard. Stir the mixture gently to blend the flavors.
Slowly pour the oil over the soy milk mixture, allowing it to settle on top. Let the ingredients sit undisturbed for about a minute to help with emulsification.
Place the immersion blender at the very bottom of the jar and blend without moving it for approximately 15 seconds, allowing the mixture to thicken. Once it begins to emulsify, gradually move the blender up and down to fully incorporate the oil and achieve a creamy consistency.
Taste the mayo and adjust seasoning if needed. Transfer to an airtight container and refrigerate. The mayo will continue to thicken as it chills. Use as desired and enjoy!