Dave Yaras was born in Chicago. As a young man he became friendly with Jack Ruby and Lenny Patrick. Yaras later worked as a hit man for Mafia boss Sam Giancana. Yaras was also a close associate of Jimmy Hoffa, president of the Teamsters Union and helped establish Teamster Local 320 in Miami.

After the Second World War Yaras worked for the Mafia in Cuba. According to a federal narcotics officer, Yaras "ran a number of gambling operations on the island". After the fall of Fulgencio Batista, Yaras was the "Chicago mob's liaison to the Cuban exile community".

Yaras, considered to be the prime suspect in several gangland killings, was arrested 14 times by the police. In 1947 he was arrested for the murder of James M. Ragen, the national manager of the Continental Press Service, an organization that was in conflict with Mafia leader, Carlos Marcello. As G. Robert Blakey and Richard Billings have pointed out in their book, The Plot to Kill the President: "Four witnesses identified Lenny Patrick, Dave Yarras, and William Block as the gunmen, but after one witness was murdered, two recanted, and another fled, the indictment was dropped." Yaras was released and no charges were ever brought against him for the killing. When the Kefauver Senate Committee later investigated the murder of Ragen, something they regarded as a landmark syndicate event), another key witness in the case was murdered.

In their book, Deadly Secrets (1992), Warren Hinckle and William Turner argue that the McClellan Senate Rackets Committee "credited" Yaras with playing a significant mob role in Havana. The historian, David Kaiser, pointed out in The Road to Dallas (2008) that Yaras was linked to Sam Giancana: "Bugs and phone taps revealed his associations with hit men Lenny Patrick and David Yaras of Chicago (both childhood friends of Jack Ruby)."

In 1962 an electronic eavesdropping device installed in a Mafia hangout by the FBI picked up a conversation where Jackie Cerone commissioned Yaras to murder Frank Esposito. Yaras said on the tape: "Leave it to us. As soon as he walks in the door.Boom! We'll hit him with an ax or something. He won't get away from us." According to a FBI informant, Yaras and Lenny Patrick were responsible for the killing of City Alderman, Benjamin F. Lewis on 13th February, 1963. One official report stated that Yaras was one of "more than a score of top-rated exterminators who work strictly on contracts for the board of directors."

The night before the assassination of President John F. Kennedy, Yaras telephoned another hit man, Robert Barney Baker. A few days earlier, Jack Ruby also received a 17 minute telephone-call from Baker. Yaras was interviewed by the FBI and admitted that he had known Ruby for about 15 years in Chicago. However, as Bernard Fensterwald has pointed out: "The FBI never asked Yaras about his own Mafia connections, but did ask him whether he thought Ruby was connected with the syndicate. Yaras, as one might guess, stated that he doubted that Ruby had such connections."

Jack Ruby's sister, Eva Rubinstein Grant, told the Warren Commission that Yarras and Lenny Patrick, were two of his closest friends in Chicago. This evidence was ignored and General Counsel J. Lee Rankin told Commission members that Ruby only had links to "the minor underworld".

According to Peter Dale Scott, the Warren Commission covered-up Ruby's connections with Yaras. In his book Deep Politics and the Death of JFK (1993) he wrote: "The Commission did not receive an important interview with Luis Kutner, a Chicago lawyer who had just told the press (correctly) about Ruby's connections to Chicago mobsters Lennie Patrick and Dave Yaras. All the FBI transmitted was a meaningless follow-up interview in which Kutner merely said he had no additional information. Apparently the FBI also failed to transmit a teletype revealing that Yaras, a national hit man for the Chicago syndicate who had grown up with Ruby, and who had been telephoned by one of Ruby's Teamster contacts on the eve of the assassination, was about to attend a 'hoodlum meeting' of top East and West Coast syndicate representatives, including some from the 'family' of the former Havana crime lord Santos Trafficante."

Dave Yaras was murdered in 1974. Lenny Patrick gave a deposition in 1978. He claimed he did not have any relevant information on Jack Ruby. However, he did accept that his friend, Yaras, "was closer to Ruby than he was".