In 1999 as a writer, director and actor, Offer released The Underground Comedy Movie to less-than-favorable reviews. The New York Post said it "may be the least amusing comedy ever made," and Lawrence Van Gelder of The New York Times described the movie as "a series of sketches built around subjects like masturbation, defecation, alienation, urination, necrophilia, voyeurism, casual brutality, and mockery of the unfortunate." He added that Offer "makes the common mistake of equating the recognition of comic potential for comedy itself. For the successful, talent bridges the gap, but, here, it is absent." DVDs of the film were marketed via television infomercial.

The film also led to several lawsuits. Offer filed a copyright-infringement suit against Peter and Bobby Farrelly and Twentieth Century Fox, claiming that 14 scenes in There's Something About Mary were taken from his own film. The Farrelly brothers responded, "We've never heard of him, we've never heard of his movie, and it's all a bunch of baloney." The case was dismissed with prejudice on a motion for summary judgment by order of the court in 2000, and Twentieth Century Fox was awarded $66,336.92 in attorneys' fees. In addition, Offer successfully sued Anna Nicole Smith for breach of contract, alleging that Smith had agreed to appear in The Underground Comedy Movie but backed out, claiming it would hurt her career.

In 2004, Offer, an ex-Scientologist, sued the Church of Scientology with the help of attorney Ford Greene. He alleged that the church had declared him a criminal and had urged its members to commit libel against him. Offer claimed that the church's actions caused him to lose a successful business, as many of his employees were Scientologists who quit upon learning of what the church did.

In July 2010 it was revealed that Offer had completed filming of brand new scenes to be included in an updated version of his 1999 movie. The Underground Comedy 2010 trailer showcases many of the same skits from its predecessor with additional skits filmed in 2010 with actress Lindsay Lohan and model Joanna Krupa. The production company, Square One Entertainment, said its film is slated for a fall 2010 theatrical release.

Offer, who funded The Underground Comedy Movie with his own money, had trouble selling it. Inspired by the success of Girls Gone Wild marketing, he decided to put the trailer on an infomercial, and sold 50,000 copies via direct order.

The success of The Underground Comedy Movie infomercial suggested to Offer that he should sell something more mainstream. Drawing upon his experience working in flea markets, he decided in 2006 to market a cleaning product that he saw there, an absorbent towel that he called the ShamWow!.

The advertisement, filmed in the summer of 2007 with a budget of $20,000, became a popular hit. Slate.com's Seth Stevenson praised Offer for his "impressive and subtle mastery of the pitchman's art" (with lines such as "you know the Germans always make good stuff"), and wondered if Offer's "abrasive manner might also mark a unique, new strategy in the annals of pitchdom." Stevenson compared Offer to earlier, "more upbeat" television pitchmen like the late Billy Mays and the Home Shopping Network hosts and concluded that Offer's "smooth-talking condescension" was more suited to the present "zeitgeist" than the "earnest fervor" of spokesmen like Mays and Ron Popeil.

Consumer Reports reported that the infomercial for ShamWow! initially featured Offer claiming that the product held "20 times its weight in liquid". Later, the infomercial was changed to Offer claiming the ShamWow! held "12 times its weight in liquid", then again to "10 times". Consumer Reports did its own test on the product and found that it does indeed hold 10 times its weight in liquid but no more.

Offer says that he has sold millions of the towels. Notwithstanding his infomercial success, Offer says that "this is not my career" and that he considers himself to be in the film business.

Pitchman Billy Mays had been promoting a similar product called Zorbeez two years prior to Vince Offer's ShamWow! product. Mays noted that the ShamWow! commercials use many of the same product demonstrations as the earlier-produced Zorbeez commercial. In February 2009 while on the Adam Carolla radio show, Billy Mays publicly challenged Offer to a "pitch off" between their respective absorbent towel products. Popular Mechanics compared the absorbency of two towel products and declared ShamWow! the more effective of the two. This result had been hotly contested by Billy Mays before his death, and was a major part of the season finale of his Discovery Channel show PitchMen. During the episode, it was suggested that the Popular Mechanics tester did not use the Zorbeez correctly. Fox's "Deal or Dud" segment tested the ShamWow and struggled to get the product working as advertised, calling it a "dud."