Omar Mir Seddique Mateen was born on November 16, 1986, at Long Island Jewish Medical Center in New Hyde Park, New York. His parents were Afghan immigrants. His father, Mir Seddique Mateen, came from Herat and moved to the United States in the 1980s, later becoming a naturalized citizen in 1989. Omar's mother, Shahla Mateen, had her own troubles, including an incident in 2002 when she was taken into custody for attacking her husband.

Omar was the first of four children in his family. They lived in New York for a few years before relocating to Port St. Lucie, Florida, in 1991. His family was described as moderate Muslims, integrating into American culture.

In school, Omar showed signs of having behavioral problems from a young age. He attended elementary and middle school in St. Lucie Count he was moved to a different class to avoid conflicts with other students. Despite these issues, his parents did not appear to take his behavior seriously.

Omar faced significant challenges in his academic journey. He began high school at Martin County High School but was expelled at age 14 for fighting in class. He subsequently attended Spectrum, an alternative school for students with behavioral issues. Reports indicated he cheered for the hijackers during the September 11, 2001, attacks and made statements bragging about connections to Osama bin Laden.

Throughout his high school years, he continued to get into trouble. He was involved in many fights and was suspended multiple times. He graduated in 2003 from Martin County's Stuart Adult Vocational School after accumulating 48 days of suspension.

After high school, Omar attended Indian River State College and earned an associate degree in criminal justice technology in 2006. He worked various jobs and, in 2006, became a recruit with the Florida Department of Corrections but was dismissed after making a concerning comment about bringing a gun to training. He later worked for the security firm G4S from 2007 until 2016.

Omar married twice. His first marriage was in April 2009 to Sitora Yusufiy, but they divorced just a few months later. He married for the second time in 2011 to Noor Zahi Salman.

On June 12, 2016, Omar Mateen entered the Pulse nightclub in Orlando, Florida, at around 2:02 a.m. armed with a SIG Sauer SIG MCX semi-automatic rifle and a Glock 17 handgun. He started shooting patrons who were inside, killing 49 people and injuring 53 others.

During the attack, Mateen made a 911 call where he pledged allegiance to the Islamic State (IS) and referenced other terrorist acts and individuals. He claimed to be acting in response to U.S. military actions against Muslims. Throughout the incident, he engaged in communication with the police and made calls to local news stations, identifying himself as the shooter and reiterating his allegiance to IS.

Around 5:14 a.m., law enforcement officers arrived and exchanged gunfire with Mateen. The shooting lasted approximately three hours before police killed him inside the nightclub. The attack was noted as the deadliest mass shooting in American history at that time and the largest act of violence against the LGBT community in the U.S.

Following the event, Mateen was buried in a Muslim cemetery in Florida, and an autopsy revealed he had been shot multiple times by police. The incident prompted discussions about gun control, terrorism, and hate crimes in America.