The ad was released by the Log Cabin Republicans, an organization for LGBT conservatives.
In the video clip, Grenell -- who is the first openly homosexual Cabinet member in United States history -- commended Trump and attacked Joe Biden, the Democratic Party's presidential nominee.
"President Trump is the most pro-gay president in American history. I can prove it," Grenell said.
"As a United States senator, Joe Biden said gay people couldn't receive security clearances because we would be a security risk," he continued.
"The fact that I'm gay didn't even faze Donald Trump. Joe Biden certainly didn't congratulate the appointment or even acknowledge it. But his silence was deafening."
While putting on a rainbow MAGA hat, Grenell concluded that "tens of thousands" of LGBT conservatives now realize they don't have to vote for Democrats because Trump is "the most pro-gay" commander in chief in history.
As reported by Fox News, both Biden and Trump have dramatically changed their views on the LGBTQ community.
In the 1970s, when he was first elected to Congress, Biden described gay Americans as a "security risk." In 2006, he argued that marriage between same-sex partners needs to be left to the states. In 2008, during the presidential campaign, he expressed opposition to same-sex unions.
However, four years later, during an appearance on Meet the Press, Biden came out in support of gay marriage. At the time, Barack Obama was opposed to it and his administration reportedly tried to walk back Biden's comments.
During the 2020 campaign, Biden repeatedly claimed that he was "the first major leader" to express support for same-sex couples. However, fact-checks established that other prominent figures did so before him. For example, his predecessor Dick Cheney spoke out in support of gay rights during the 2000 vice presidential debate.