Whisk together soy sauce, sherry, and sugar in small bowl.
Heat oil in a wok or large non-stick skillet over medium-high heat. Add mushrooms and stir-fry for 2 minutes. Add shrimp, garlic, and ginger. Stir-fry for 1 minute.
Add spinach and soy mixture and continue stir-frying until spinach has just wilted (shrimp will be cooked), about 1 minute.