L.A. Noire is a hard-boiled detective game set in Los Angeles of 1947. Drawing strongly from the stylistic film noir, players control LAPD officer Cole Phelps through 21 cases as he works his way through the ranks of his department to solve a series of murder mysteries. Each case forms an independent section in the game, but there is an overarching story with recurring characters and Phelps's personal history while serving during the Second World War. He starts off as a patrolman, but eventually advances to a traffic detective, homicide, vice, and finally arson investigator. With each promotion, a new partner and new locations become available.

During a case the game blends slower investigative elements with fast sequences that involve on-foot and car chases, and gun-play. There is a strong focus on the story and character development, often with long cinematic sequences. Even though the game offers a large amount of freedom with exploration by car and on foot, it cannot be compared to Grand Theft Auto due to a number of restrictions. Playing as a detective, random civilians cannot be attacked and gun-play is reserved for specific sequences. In addition, each case is confined to a limited number of venues. The player is free to choose the order of investigation, but ultimately the game cannot be furthered until a satisfying conclusion for the case has been reached. Next to the main story Phelps can accept optional side-investigations through dispatch calls.

When arriving on the scene of a murder case, it needs to be hunted for clues that can be researched. The game offers hints through rumble and audio chime when Phelps walks by, but these can also be disabled. Armed with evidence, the next step is usually interrogating witnesses and suspects. Faces of characters have been animated through the motion capture MotionScan technology and the detailed realism of these captures is used as an element of gameplay. When talking to characters, not only the evidence is important, but also their tone of voice, and especially facial expressions. Characters will sometimes look away, or have tics and expressions that can reveal information. During an interrogation the player needs to interpret these and then decide how to proceed through one of three options: Truth, Doubt, or Lie. Based on the decision, different paths and reactions open up. Failed action sequences can be replayed, but when the player constantly interprets the situation incorrectly and the investigation grinds to a halt, the entire case needs to be replayed. Plenty of chances are offered to correct mistakes and try a new approach, also often after the investigation of clues.

Each case takes about an hour to complete. Depending on the speed and correct intuition applied to solve cases, the player earns ranks that provide intuition points. These can be used to eliminate options during conversations when stuck.