Halpern grew up in the Point Loma neighborhood of San Diego, California. His father is Jewish and his Italian American mother is Catholic. His father, Samuel Halpern, M.D., is a radiologist, now retired, who practiced at the University of California San Diego. A blunt-spoken man, he gave memorable, expletive-laden advice and comments to young Justin from childhood on. Samuel Halpern has been described as "a profane comic genius," "Lenny Bruce with a stethoscope," "a scatological Socrates showering rough wisdom on his son."

Halpern graduated from San Diego State University in 2003. He then headed for Hollywood looking for work as a writer. He wrote screenplays and waited tables, but success as a screenwriter eluded him. By 2009 he was a full-time writer for Maxim Magazine's online site and no longer needed to live in Los Angeles. He moved back to San Diego and wound up living with his parents.

Halpern had always kept a record of his father’s salty comments. Some, repeated to him by brothers, cousins and other relatives, go back to when he was 4 or 5 years old. When he started living with his parents, his record of his father’s remarks became a daily journal. On August 3, 2009, he started a Twitter feed, @shitmydadsays, just to have a place to store his father’s rhetorical gems. He thought he might use some of them someday in a script. On the Twitter site he explains, "I'm 29. I live with my 74-year-old dad. He is awesome. I just write down shit that he says."

To his surprise he quickly gathered a large following on Twitter. A friend posted a link to his feed, then Rob Corddry tweeted it, and "that really jump-started it" according to Halpern. By mid-August he had more than 100,000 followers. By October he had a book deal with Harper Collins, and by November he had a television deal with Warner Bros. As of May 2010 his Twitter feed had 1.3 million followers.

He and his longtime writing partner Patrick Schumacker completed the book, with the title Sh*t My Dad Says, in February 2010 and it was published in May. In June it topped the New York Times best seller list.

The television show is called $#*! My Dad Says (pronounced "Bleep My Dad Says") with William Shatner in the title role as the colorful curmudgeon. The pilot, written by Halpern and Schumacker, was filmed in March 2010, and CBS picked it up in May. Halpern says the series is about "the dichotomy of this older guy who says whatever he wants and this younger guy who is tiptoeing through life," careful not to offend for fear of losing jobs or friends.