McConaughey spent the opening minutes of the briefing talking about those who died, saying he needed to tell their stories to show how action needed to be taken to honor the lives of the 19 children and two teachers killed at Robb Elementary School last month in his hometown of Uvalde, Texas.
"We are in a window of opportunity right now that we have not been in before. A window where it seems like real change, real change can happen," McConaughey said from the podium.
MORE: What we know about 21 Uvalde school shooting victims
McConaughey said, "I'm here today in the hopes of applying what energy, reason and passion that I have into trying to turn this moment into a reality. Because as I said, this moment is different."
McConaughey held meetings with lawmakers on Capitol Hill earlier in the day to discuss gun reform legislation.
The White House said McConaughey would join White House press secretary Karine Jean-Pierre at Tuesday's briefing and deliver remarks.
The Academy Award-winning star of "Dallas Buyers Club" publicly weighed a run for governor in Texas last year but ultimately ruled one out -- for now. He said it was "a path that I'm choosing not to take at this moment."
McConaughey told reporters on Capitol Hill on Tuesday he hoped he was making progress in his meetings with lawmakers. He had left a meeting with House Speaker Nancy Pelosi and was heading to another meeting with House Minority Leader Kevin McCarthy.
SEE ALSO: 9-year-old says teacher played dead after being shot in Uvalde classroom, recounts seeing gunman
The President earlier on Tuesday met with Democratic Sen. Chris Murphy of Connecticut, who is leading bipartisan negotiations on gun reform. Murphy told reporters at the White House he met with the President for about half an hour and spoke about the outlines of the ongoing gun reform negotiations. Murphy emphasized how much he appreciated Biden and the White House giving senators "space" to try and reach a deal, and said his goal remains to reach a deal by the end of this week.
While the odds of any sweeping reforms remain very steep, lawmakers have expressed optimism that a deal for narrow and targeted bill could be reached as soon as the end of this week. Jean-Pierre said Monday Biden was "encouraged" by the Senate negotiations on gun control measures.
The President delivered an impassioned speech from the White House after the mass shooting at the Uvalde elementary school last month and ratcheted up the pressure on Congress to act. He has called on Congress to implement stricter gun laws, including a ban on assault weapons, tougher background check laws and a higher minimum age of purchase.
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The number of people injured or killed does not include the suspect or perpetrator. These graphics show the number of victims across all mass shootings from the last five years.