And Houston Texans star defensive end J.J. Watt tweeted his support for the move Saturday, dismissing a notion from US President Donald Trump that kneeling is disrespectful to the US flag and military.
The social media support followed a week in which NFL commissioner Roger Goodell said the league was wrong not to more fully embrace the "Black Lives Matter" movement and oversaw a $250 million donation over 10 years to battle systemic racism.
When a fan sent Mayfield an Instagram message saying, "Please tell Browns fans you're not going to be kneeling this season," Mayfield replied, "Pull your head out. I absolutely am."
Mayfield had already been supportive of the need for more social justice and a greater understanding of racial issues in the wake of the killing of George Floyd while in police custody, which set off protests worldwide.
Watt, a five-time NFL Pro Bowl defender, fired off a response to a tweet suggesting he wouldn't kneel during the anthem.
"Don't speak for me," Watt said. "If you still think it's about disrespecting the flag or our military, you clearly haven't been listening."
Watt cited a narrative that became prominent in 2017 when US President Donald Trump made NFL kneeling protests a major issue, saying they insulted the US flag and military.
Trump called any players kneeling during the flag "sons of bitches" and said they should be fired.
Former San Francisco 49ers star quarterback Colin Kaepernick, who began the kneeling protest in 2016, has not played in the NFL since that season.
While Watt didn't say he would kneel, Texans coach Bill O'Brien said Friday that he would join players who wanted to kneel in protest of police brutality during the national anthem.
"The players have a right to protest, a right to be heard and a right to be who they are," O'Brien said. "They're not taking a knee because they're against our flag. They're taking a knee because they haven't been treated equally in this country for over 400 years."
New Orleans Saints quarterback Drew Brees apologized for comments saying he would never agree with anyone disrespecting the US flag after drawing a firestorm of objections, saying, "We can no longer use the flag to turn people away or distract them from the real issues that face our black communities."