Scoop avocado out of peels and place in a wide-mouthed quart jar. Add lemon juice, salt, and oil.
Place immersion blender in jar and process until thickened and completely emulsified.
Transfer mayonnaise to serving container. Serve as soon as possible after making.
Store leftover mayonnaise in covered container and refrigerate. The lemon juice slows oxidation of the avocado, but the mayonnaise can begin to turn brown after 1/2 to 1 day.