Nancy Mendenhall is a grade III Detective with the Los Angeles Police Department's Professional Standards Bureau. The PSB is the successor to the Internal Affairs Division. Her office is located in the historic Bradbury Building.
In 2012, she was assigned to investigate detective Harry Bosch's visit to convicted rapist Shawn Stone at San Quentin State Prison. She met with Bosch and Rick Jackson in her office. She also interviewed Hannah Stone, and in that conversation Mendenhall mentioned that she was "considering the motivation behind the original complaint" against Bosch. This refers to the fact that Stone had made the complaint about Bosch. She seemed inclined to view it as a bogus complaint, but she was obligated to follow through with the investigation. She went as far as visiting Stone in San Quentin to get his statement.
When Bosch went to Stanislaus County, ostensibly on vacation, she believed there was something more going on. She followed him there and found him locked in a barn on Carl Cosgrove's estate, with the body of Reggie Banks also inside. They searched the house, finding the bodies of Francis Dowler and Cosgrove. As J. J. Drummond attempted to escape by helicopter, she called for paramedics and backup. She later dismissed the 128 complaint and told Bosch that Stone had filed the complaint against him.
In early 2015 she came into contact with Bosch again in connection with the murders of Lexi Parks, the Nguyen brothers, Dr. Schubert, and the wounding of LAPD vice detective Kevin Long. She was called into an interview with Bosch because Internal Affairs was already interested in the activities of Long and his partner. After the interview she gave Bosch a ride to his house and asked him why he hadn't call her after their adventure in Modesto. As she was leaving she saw the car belonging to Don Ellis. She returned to Bosch's house where she found Ellis holding Bosch at gunpoint. She shot Ellis once in the head, killing him and saving Bosch's life for the second time. Afterwards she and Bosch talked about getting together later.