Jimmy Conway is an Irish-American associate of the mafia. Since he has no Italian blood, he cannot become an official member of the mafia (made man). Despite this, he performs jobs for caporegime Paul Cicero such as robberies and hits. Jimmy's close friends are the psychotic Tommy DeVito and the quiet Henry Hill. Early on in the film, a younger Jimmy talks to the teenage Henry after he has been arrested and charged for smuggling illegal cigarettes. Jimmy is not angry at Henry, but is rather proud of the fact he was loyal. According to Jimmy, the two most important things in life are "you never rat on your friends and you always keep your mouth shut".
One of the most brutal murders that Jimmy took part in was the murder of Billy Batts in 1970. After Billy had insulted Tommy, Jimmy helped Tommy to beat Billy to a bloody pulp. Later on, Billy woke up in the trunk of Henry's car. Jimmy finished Billy off by emptying his gun into him. Shortly after the Batts murder, Jimmy ordered Henry and Tommy to exhume Batts' corpse as the land where they buried him was to be used to build houses on. Being a hardened criminal, the smell of Batts half-decomposed body did not bother Jimmy, but it made Henry vomit.
One night, Jimmy was playing cards with his friends when Tommy got into an argument with the teenage bartender Spider. Tommy shot Spider in the foot, which horrified Jimmy. Shortly afterwards, Spider insulted Tommy by telling the latter to "go fuck himself". Jimmy was amazed at Spider, but was furious when Tommy shot Spider dead.
Jimmy accompanied Henry to Miami to beat up a man who owed Paulie money. Jimmy was going to feed the man to the lions when he promised to pay them. Unfortunately, the man turned out to have a sister working as a typist for the FBI. Jimmy and Henry were arrested and were each sentenced to ten years in prison. It is not known what prison life was like for Jimmy as he served his time in a different prison from Henry. He was released prior to 1978.
After being released from prison, Jimmy organised a major heist at Lufthansa airport. They stole $5 million dollars. Although Jimmy gave each person a share of the money, he got very angry when they started to buy expensive stuff. This was because Jimmy feared that lavish spending would draw attention to them. It is also evident that Jimmy wanted to keep most of the money for himself, so he had each person involved in the heist brutally murdered.
Jimmy was really happy to hear that Tommy was to become a made man. This would mean that they had one of their own crew as an official member of the mafia. Unfortunately, the ceremony was a set-up where Tommy was murdered to compensate for the unauthorised murder of Billy Batts several years previously. This made Jimmy cry. Henry lamented that in order to kill a made man, you required to have permission from his boss.
A year after Tommy's murder, Jimmy started to lose patience with Henry as he was involved in the drugs business in the worst way possible. Dealing in drugs was strongly discouraged within the mafia as it was long considered to be a dangerous business. Henry was also taking cocaine which was making him act stupid and was bringing police attention on him. Jimmy tried to sever ties with Henry, but when it looked that Henry was going to prison, he entered the witness protection scheme if he agreed to testify against Jimmy. This is because Jimmy would have Henry killed if he was in prison or outside. Jimmy was sentenced to life in prison for murder. He was given a minimum sentence of 20 years, which would have made him eligible for parole in 2004. However, in real life, Jimmy died in prison in 1996.
In contrast to fellow mobster Tommy DeVito, Jimmy is shown to be calm. However, he is equally as violent and dangerous as Tommy and has no qualms about killing. Jimmy shows his most dangerous side when he starts getting paranoid at his friends for overspending money as he does not want any heat. Deep down, Jimmy is vain and selfish as he only wants the big money for himself. His softer side is shown as he cries when his best friend Tommy is murdered.