Much of the humor comes from the contrasting personalities of the brash Italian American Vinny and his fiancée Mona Lisa, and the more laid back Southern townspeople. Pesci and Tomei received vast critical praise for their performances, and Tomei won the Academy Award for Best Supporting Actress. Fred Gwynne, until then typecast as Herman Munster in The Munsters, also received critical acclaim for his performance as Judge Chamberlain Haller.

Despite being a comedy, the film also touches on some rather serious subjects such as perjury, contempt of court, mistrials and bias.

In the August 2008 edition of the American Bar Association Journal, My Cousin Vinny was rated #3 in their cover story listing of "The 25 Greatest Legal Movies", after To Kill a Mockingbird and 12 Angry Men.

While driving through the fictional Beechum County, Alabama, New Yorkers Billy Gambini (Ralph Macchio) and his friend Stan Rothenstein (Mitchell Whitfield) accidentally neglect to pay for a can of tuna after stopping at a convenience store. After they leave the store, the clerk is shot and killed, and Billy and Stan, who match the descriptions of the murderers given by witnesses, are then pulled over and detained in connection with the murder. Due to circumstantial evidence and a series of miscommunications based on the boys’ assumption that they have merely been detained for shoplifting, Billy ends up being charged with murder, and Stan is charged as an accessory. The pair call Billy's mother, who tells her son that there is an attorney in the family, Billy's cousin, Vincent LaGuardia "Vinny" Gambini (Joe Pesci), who travels to Beechum County accompanied by his fiance, Mona Lisa Vito (Marisa Tomei). Unfortunately, although he is willing to take the case, Vinny is a neophyte personal injury lawyer from Brooklyn, New York, newly admitted to the bar (after six attempts and six years) with no trial experience whatsoever.

Although Vinny manages to fool the trial judge, Chamberlain Haller (Fred Gwynne), about being experienced enough to take the case, his ignorance of basic court procedures and abrasive, disrespectful attitude towards the judge gets him into trouble immediately. Much to his clients' consternation, Vinny does not even bother to cross-examine any of the witnesses in the probable cause hearing. As their claims go unquestioned, it appears that the prosecution has an airtight case that will inevitably lead to a conviction at the trial. After Vinny's poor showing at the hearing, Billy and Stan decide to fire him and use the public defender, but Vinny asks for one more chance to prove himself. The trial then opens with Vinny representing his cousin and the public defender representing Stan. Despite some further missteps, including wearing a gaudy secondhand suit to court (his new suit fell in the mud) and sleeping through the District Attorney's opening statement, Vinny shows that he can make up for his ignorance and inexperience with an aggressive, perceptive questioning style. While the public defender stutters through a line of ill-prepared questions that appear to bolster the case against the boys, Vinny quickly and comprehensively discredits the testimony of the first witness. Billy's faith is rewarded, and Stan develops newfound respect and confidence for Vinny.

Vinny's cross-examinations of the remaining eyewitnesses are similarly effective, but the DA produces a surprise witness, George Wilbur, an FBI analyst who testifies that his chemical analysis of the tire marks left at the crime scene shows that they are identical to the tires on Billy's Buick Skylark. With only a brief recess to prepare his cross-examination and unable to come up with a particularly strong line of questions, Vinny becomes frustrated and lashes out at Lisa by taunting her about the usefulness of her wide-angle photographs of the tire tracks. She storms out, leaving Vinny alone. However, he later realizes that that photo actually holds the key to the case: the flat and even tire marks reveal that Billy's car could not have been used for the getaway. Vinny needs Lisa, an expert in automobiles, to testify to this. He drags her into court, and during Vinny's questioning, they patch up their differences. Vinny then recalls the FBI analyst, who concurs with Lisa that Billy's car did not produce the tracks. Next, Vinny calls the local sheriff, who has run a records check at Vinny's request. The sheriff testifies that two men resembling Billy and Stan were arrested driving a stolen Pontiac Tempest and in possession of a gun of the same caliber used to kill the clerk. The DA then moves to dismiss all the charges.

Throughout the film, Vinny and Judge Haller play a game of cat-and-mouse over Vinny's qualifications. Haller first discovers that, despite Vinny's claims that he tried "quite a few" murder cases, there exist no records of anybody named Vincent Gambini trying any case in New York State. Vinny then claims that he had his name changed during a previous career as a stage actor and continued to use the name when he opened a law practice. Vinny, believing that he should give the judge the name of someone with the kind of resume he claimed to have, supplies the name of a prominent New York attorney, Jerry Gallo. Unfortunately, Lisa reveals the source of Gallo's most recent publicity: he died the week before. Vinny then claims that Haller misheard "Gallo" when Vinny actually said "Callo". Finally, Lisa gets Vinny off the hook by calling his mentor, a judge from New York, who responds to Haller's request by claiming that Jerry Callo has a long and impressive trial history. The film concludes with Haller apologizing for doubting Vinny and praising his skills as a litigator. Vinny and Lisa then drive off together, discussing how they were able to fool the judge about Vinny's qualifications and arguing about plans for their wedding.

Sac-O-Suds convenience store is located on Georgia State Route 16 in Jasper County, Georgia, which is east of Jackson, Georgia. The courthouse is still in the town square of Monticello, Georgia. Dave's Barbecue and Sea Food (where Vinny and Lisa get in a fight near the end of the movie) is adjacent to the town square.

Shooting took place in and around Jasper and Putnam County, Georgia. Putnam General Motel where they first stay the night and then learn about grits in the morning is located on Highway 441 North in Putnam County outside of the town of Eatonton. The motel is still open but the restaurant is now closed and has been for a few years. The actual lumber plant that is supposed to be across the street from the Motel in the movie is actually the Georgia Pacific Plant in Jasper County Georgia. Sheriff Farley mentions that the two boys arrested in the Tempest were arrested in Jasper County, Georgia. Sheriff Farley also mentions Sheriff Tillman of Jasper County Georgia as being the Sheriff that did the arrest. Sheriff Tillman was the actual Sheriff of Jasper County Georgia at the time of the movie.

The film received generally critical acclaim by critics, holding a score of 86% with 32 positive reviews out of 37 on Rotten Tomatoes. Jonathan Rosenbaum, critic for The Chicago Reader wrote of it "While it's easy to imagine an infinite number of bad courtroom comedies based on this scenario, this movie turns out to be wonderful -- broad and low character comedy that's solidly imagined and beautifully played." Roger Ebert for the Sun Times had a less positive review, saying "It's the kind of movie home video was invented for: Not worth the trip to the theater, but slam it into the VCR and you get your rental's worth." Writing for the Washington Post, Desson Thomson lauded Joe Pesci, stating that "Joe Pesci is a total screen pleasure. He grabs whatever comes his way -- from GoodFellas to Home Alone"