If the tree is within the city limits of Mill Valley and is on City property you should contact the Mill Valley Parks and Recreation Department at 415-383-1370 with the location of the tree or limbs.
If the tree is within city limits and is blocking a public right-of-way or roadway contact the Mill Valley Public Works Department at 415-384-4800 (Monday through Friday 9 am to 5 pm) and after-hours call the Mill Valley Police Department at 415-389-4100 with the location of the tree or limbs.
If the tree is on private property and has fallen onto private property contact the property owner.
If the tree is touching electrical wires contact PG&E at 800-743-500 or the Mill Valley Fire Department at 415-389-4130.
If the tree is outside of the city limits but is on county property contact the County of Marin Public Works at 415-499-7877.