Griner has been freed from a Russian penal colony after the United States made a prisoner exchange for her following her February detention and August conviction on drug charges.
"This was the best news to wake up to today. I can't stop crying!" the New York Liberty's Sandy Brondello, who coached Griner for eight seasons with Phoenix, told ESPN. "It has been a terrible ordeal for BG, but thankful for President Biden and Vice President [Kamala] Harris and everyone that has helped get her home.
"So happy for [Griner's wife] Cherelle and BG's family that she is finally back where she belongs."
South Carolina coach Dawn Staley, who coached Griner at the Tokyo Olympics, also had an emotional reaction to Thursday's news.
"I was shocked actually ... and then, honestly, I cried," Staley said on ESPN's "First Take." "I cried, and I'm a girl from Philly. We don't cry very easily. But I cried because this is the moment that we've all been praying for, for BG. And I'm just happy, like God is real."
The Mercury won the WNBA title in 2014 with Brondello at the helm and Griner, their No. 1 overall pick in 2013, as one of the team's key players. They also competed in the 2021 WNBA Finals, falling to the Chicago Sky.
"Miraculously, mercifully, the count of days detained has ended at 294, and our friend, our sister is headed back home where she belongs," the Mercury and Phoenix Suns said in a joint statement. "The emotions for our organization, just like for our fans and so many across the world, are those of joyous celebration, deep gratitude, grief for the time lost, and sincere hope for all families still awaiting the return of a loved one."
Prior to her WNBA career, Griner was an All-American at Baylor and led the Bears to the 2010 Women's Final Four and 2012 national championship, when they went 40-0.
"God is good. Prayers are powerful," LSU's Kim Mulkey, who coached Griner at Baylor, told ESPN. "Brittney is on her way home where she belongs. Our prayers remain with her and her family as they recover and heal together."
Like many WNBA players, Griner, 32, played overseas. In February, she was headed to Russia to rejoin her UMMC Ekaterinburg team when she was detained in Moscow's Sheremetyevo Airport for having vape cartridges that contained cannabis oil in her luggage.
Since then, the WNBA, its players' union and the NBA have worked with the State Department to try to get Griner back to the United States.
"There has not been a day over the past ten months where we all haven't had Brittney Griner on our minds and in our hearts," WNBA commissioner Cathy Engelbert said in a statement. "And that has now turned into a collective wave of joy and relief knowing that she will soon be reunited with her family, the WNBA player community, and her friends."
The WNBA players' union released a statement thanking the Biden administration and others for helping Griner.
"Whether you signed the petition, posted on social media, wrote a letter, wore a shirt, told your friends to advocate, or called our community to action -- what you did made a difference," the union said. "Your actions kept BG's name top of mind and her humanity in focus. BG is a daughter, a sister, a wife, a friend, a teammate, and you showed up for her."
And Las Vegas Aces coach Becky Hammon tried to sum up the importance of Griner's return in an interview on "SportsCenter."
"Women's basketball is a small community, and it just didn't feel whole without BG in it," Hammon said.
ESPN's Alexa Philippou contributed to this report.