Preheat the oven to 350 degrees F (175 degrees C).

Bring water to a boil in a saucepan. Cook macaroni in boiling water, stirring occasionally, until tender, 7 to 8 minutes; drain.

Return macaroni to the saucepan. Stir cheese sauce mixture from the packet, butter, and milk into the macaroni until the butter melts and the macaroni is evenly coated. Stir in tuna and mushrooms. Pour macaroni mixture into a small casserole dish; top with bread crumbs.

Bake in the preheated oven until heated through and bread crumbs are lightly browned, about 30 minutes.