Seddique Mateen, whose son Omar killed 49 people at Pulse nightclub on June 12, sat with a smile on his face behind the Democratic presidential nominee.
The former secretary of state started her address to the crowd on Monday night in Kissimmee, Florida, by paying tribute to the victims of the horrific terrorist attack that shocked the world two months ago.
Mateen, a pro-Taliban political activist, later said he was there because he's a member of the Democrats and had been 'invited'.
On Tuesday, members of Clinton's campaign denied he was given an invitation and was not aware of his presence.
Mateen blended into the crowd of supporters as they listened to the former first lady talk about jobs and parts of her economic policy.
At some points he took out his cell phone and took pictures of Hillary while she was on stage. After the event, Mateen spoke with local TV station WPTV.
He said: 'Hillary Clinton is good for United States versus Donald Trump, who has no solutions.
'I wished that my son joined the Army and fought ISIS. That would be much better.'
It is not known whether he had been personally asked to attend, or had received the offer as party of an email chain.
When asked whether Clinton knew he was at the rally, he insisted: 'It's a Democratic party, so everyone can join.
Then he was asked if those in the crowd would be surprised if he was there.
He said: 'Why should they be surprised? I love the United States, and I've been living here a long time.