1920: Big Jim Colosimo is slain in his popular Wabash Avenue restaurant, making way for the rise of Al Capone. Largely credited with taking the steps to create what would become known as the "Chicago Outfit"
1924: Dion O'Banion is shot dead in his flower shop across from Holy Name Cathedral. Chief suspects are his beer war enemies, the Genna brothers. Started hijacking whiskey right before the start of prohibition kicked in.
1929: Seven members of the Bugs Moran gang are gunned down, allegedly on orders of Capone, at 2122 N. Clark in the infamous St. Valentine's Day Massacre. Moran himself, lucky man, is late for the meeting at the S.M.C. Carting Co.
1930: Jake Lingle, a Chicago Tribune reporter in the mob's pocket, is slain in the Illinois Central train station. He had crossed many mobsters, including Capone. Shot behind the ear with a 38 caliber detective's special on the way to the racetrack, Lingle was given a hero's funeral. It was only later that it was learned that he was really a legman for the mob.1936: Capone gunman and bodyguard "Machine Gun" Jack McGurn is gunned down at a Milwaukee Avenue bowling alley, the day before Valentine's Day. Given the timing, the Moran gang was suspected. In addition to his skill with a machine gun, McGurn was also considered a scratch golfer who considered going pro and boxed as a welterweight where he was known as Battling Jack McGurn. He is credited with over 25 mob kills and McGurn was also suspected of being the principal gunner and planner of the St. Valentines Day Massacre.
1975: Mob boss Sam Giancana is killed, while cooking sausage, in the basement of his Oak Park home after he becomes a liability to the Outfit. "The Don" calls Giancana the Godfather of Godfathers - The Most Powerful Mafioso in America. Started as a hitman for Capone. Rose to boss of the Chicago crime family. Friend of celebrities such as Frank Sinatra & Marilyn Monroe. Rigged the Chicago vote for John F. Kennedy in 1960.
1978: Six burglars who struck at mob boss Anthony Accardo's (AKA Joe Batters by the FBI and THE Big Tuna by the Chicago media) house are found slain across the city.1983: Worried he will sing to the feds, mobsters gun down crooked Chicago businessman Allen Dorfman outside the Hyatt Hotel in Lincolnwood. Dorfman had already been convicted under operation Pendorf: Pentration of Dorfman, along with Teamsters President Roy Williams and Joey "The Clown" Lombardo, when he was hit by the Outfit afraid he would look to reduce his sentence.
1983: Mob gambling lieutenant Ken Eto is shot three times in the head. Miraculously, he survives and testifies against old pals.
1986: The mob's man in Vegas, Anthony Spilotro, and his brother Michael Spilotro are beaten and buried alive in an Indiana cornfield. Glamorized in the movie Casino in which Joe Pesci played "Tony the Ant". Opened up a gift shop at the Circus-Cirus Hotel and Casino where he based his operations. The Family Secrets Trial revealed that the two were originally murdered by a crew led by James Marcello in a house in Bensonville.
2001: Anthony "the Hatch" Chiaramonti, a vicious juice loan debt collector, is shot to death outside a restaurant in suburban Lyons by a man in a hooded sweat shirt. Chiaramonti had been caught on a tape played at the trial of Sam Carlisi, grabbing a trucking company owner, Anthony LaBarbera, by the throat, lifting him in the air and warning him not to be late in paying juice loan money. LaBarbera was wearing an FBI body recorder at the time. Interesting enough, the restaurant where he was shot was a Brown's Chicken and Pasta, where I have had lunch a handful of times.
Thanks to the Chicago SunTimes and additional various sources.