The incident, which left at least 53 others injured, was described as the worst mass shooting in US history.
The assailant was said to have hailed from Port St. Lucie, Florida.
Officials said Mateen, who was armed with an assault-type rifle, a handgun among other weapons, died in a shootout with the police force at the Pulse nightclub.
Al Jazeera reports that US news networks and a Congress member identified the gunman as a New York-born resident of Florida with Afghan origins, but investigators had yet to make an official announcement.
Florida Governor, Rick Scott, declared a state of emergency following the shooting.
Orlando Police Chief, John Mina, described the shooting as one of the worst tragedies, adding that the scene was horrific.
He said, “It’s a tragedy not only for the city but the entire nation. Just a look into the eyes of our officers told the whole story.”
The injured had been transferred to nearby hospitals, police and medical officials said. Among those injured was one police officer, whose kevlar helmet was hit by a round from the suspect.
“This did turn into a hostage situation. At approximately 05.00 hours (09.00 GMT) this morning, the decision was made to rescue hostages that were in there,” Mina added.
Sky News reports a senior Federal Bureau Intelligence official as saying the assailant could have connection with the Islamic State militants.
An Islamic State-affiliated Twitter account has posted an image of a man it claims to have been the shooter, although there has been no official IS statement.
House Homeland Security Chairman, Michael McCaul, said, “This appears to be the worst terrorist attack on American soil since 9/11. It is a sobering reminder that radical Islamists are targeting our country and our way of life.”
In a statement, the White House press office said President Barack Obama was briefed of the incident by Lisa Monaco, top adviser for Homeland Security and Counterterrorism.
As the shooting occurred, the nightclub urged patrons to “get out” and “keep running” in a post on its Facebook page.
An eyewitness, who said he was inside the building during the incident, said he heard about 40 shots being fired.
A mother said her son, who was in the club, sent her a text message from the club toilets where he was hiding, telling her, “I’m gonna die. Mommy, I love you. He’s coming.”