The series has gone through a couple of title iterations, beginning with L.A. Noir (the name of the book it was based on, but also that of a video game that came later and therefore a problematic title). Then it went to Lost Angeles, which never really resonated with audiences.
Check out the official description and trailer below.
The epic battle between a determined police chief and a dangerous mobster inflames 1940s Los Angeles in TNT's eagerly anticipated television event Mob City (formerly known as Lost Angels). This powerful drama comes to TNT from Frank Darabont (The Walking Dead), who wrote and directed the pilot and serves as executive producer on the series. Based on the critically acclaimed book L.A. Noir: The Struggle for the Soul of America's Most Seductive City, by John Buntin, Mob City opens in post-war Los Angeles, home to glamorous movie stars, powerful studio heads and returning war heroes. But it's also a city caught between a powerful and corrupt police force and an even more dangerous criminal network determined to make L.A. its West Coast base. Los Angeles Police Chief William Parker (Neal McDonough -- Captain America, Desperate Housewives)) has made it his mission to free the city of criminals like Ben "Bugsy" Siegel (Ed Burns -- Entourage) and Mickey Cohen (Jeremy Luke -- Don Jon), the ruthless king of the Los Angeles underworld. Parker also won't hesitate to go after anyone from his own police force who sells out honor and duty for the sake of a big payout. To carry out his sweep of organized crime, Parker sets up a new mob task force within the LAPD. Headed by Det. Hal Morrison (Jeffrey DeMunn -- The Walking Dead, The Shawshank Redemption), the task force includes Det. Joe Teague (Jon Bernthal -- The Walking Dead), an ex-Marine who holds his cards close to his chest. Mob City's exceptional ensemble cast also includes Gregory Itzin (24), Robert Knepper (Prison Break), Milo Ventimiglia (Heroes) and Alexa Davalos (Clash of the Titans, The Mist). Mob City is produced by TNT Originals, with Darabont, Michael De Luca (The Social Network) and Elliot Webb (Alpha House) serving as executive producers.