Complex sentence: She serves as District 5 Supervisor and President of the San Francisco Board of Supervisors, and as holder of this office became acting mayor according to the city charter following the death of mayor Ed Lee on December 12,2017.
Split into simpler sentences: She serves as District 5 Supervisor and President of the San Francisco Board of Supervisors. As holder of this office, Breed became acting mayor according to the city charter following the death of mayor Ed Lee on December 12,2017.