Gentleman Johnnie Marcone (pronounced mar-CONE, contrary to Marsters' mar-COE-nee on the audio books) is Chicago's most powerful crime boss. Marcone controls the majority of organized crime in Chicago, and appears to have a growing knowledge of the supernatural. Harry loathes him, though circumstances have occasionally forced them to work together to defeat common foes. His past and motives are a mystery, though we are given small insights in Harry's soulgazes with Marcone himself in Storm Front and Miss Demeter in White Night.
In Storm Front, Marcone attempts to hire Dresden as a supernatural bodyguard. Harry flatly refuses. Later, he suspects Marcone is responsible for sales of the dangerous the ThreeEye drug, only to learn Marcone's bodyguard Gimpy had betrayed his boss and was working for the real culprit, Victor Sells. Rather than making a public enemy of Harry after the wizard trashed his club, Marcone spread the rumor that Harry took down Victor and the ThreeEye ring at his behest. Harry chose not to deny the rumor, as it was easier than the alternative.
In Fool Moon, Marcone repeats his offer, which Harry refuses again. Marcone then leaves Harry to be killed by the warring lycanthropes, but when Harry escapes, they must work together to escape the rogue FBI agents' trap.
In Death Masks, Marcone arranged the theft of the Shroud of Turin, in which Christ was reputedly buried, in order to use its legendary healing powers on a young Jane Doe he has sequestered in a hospital care facility in Wisconsin. Marcone's relationship to the girl is unknown, as is whether his use of the Shroud led to any change in her condition. He also acquires a new employee, called only "Miss Gard," who doubles as bodyguard and supernatural consultant.
In Dead Beat, Marcone and Miss Gard appear briefly to rescue Harry from a ghoul and give him a ride to the hospital.
In White Night, we learn the identity of Marcone's Jane Doe and gain an insight into Marcone's motivations. Jane Doe is the daughter of Helen Beckett, alias Miss Demeter, one of Victor Sells patrons from Storm Front. In exchange for his assistance taking down several White Court nobles, Harry offers Marcone the position of Freeholding Lord under the Unseelie Accords that partially govern the magical world, making him the only non-magical human to take such a position. The other signatories are Lara Raith, signing on behalf of her father the White King, and Donar Vadderung, CEO of Monoc Securities.