"We, her family, are devastated by her passing and unimaginable loss," Marczewski's family shared in a statement to TODAY. "Those who knew her enjoyed her larger-than-life personality and sense of humor. She had a witty joke for every occasion -- even if the joke was on her."
She also revealed at the time that she was still fighting cancer, and had been given a two percent chance of survival.
She wowed the judges with the original, uplifting tune, which detailed the past year of her life -- including her battle with cancer, and her realization that it's important for people to know that "we're so much more than the bad things that happen to us."
She took to Instagram on Aug. 2 to reveal that she chose to step down from the season and that her health had "taken a turn for the worse."
"Sharing my heart with the world on AGT has been an honor and a dream come true. My point of view this summer has been astounding," she shared in her post, alongside a somber black-and-white snapshot of herself sitting in her bed. "What a miracle that the pain I’ve walked through can be reworked into beauty that makes people all over the world open their eyes wider."
"Since my audition, my health has taken a turn for the worse and the fight with cancer is demanding all of my energy and attention," she continued. "I am so sad to announce that I won’t be able to continue forward on this season of AGT. Life doesn’t always give breaks to those that deserve it—but we knew that already."
Marczewski concluded, "Thank you for all your support, it means the world to me. Stay with me, I’ll be better soon. I’m planning my future, not my legacy. Pretty beat up, but I’ve still got dreams."
The singer's final post to social media came on Jan. 11, with a smiling selfie and an emotional caption.
"Honestly, things have been pretty brutal. But this is a photo of myself from last week where I felt pretty, and alive, and awake, and human, and real. I needed that," she wrote. "We’re all a little lost and it’s alright."
ET's Matt Cohen spoke with Cowell about the weight of Marczewski absence during the show's season finale. The outspoken judge said he'd been in contact with her ahead of the evening, and admitted that he thought she could have gone all the way and been crowned champion, if it had been possible.
"Would she have won tonight? I think she probably would, actually," Cowell said. "And I think she knows that deep down. Having said that, she knows she made the right decision."
On Tuesday, Cowell posted a tribute to Marczewski, writing, "Heartbreaking news to hear about Nightbirde, she was an extraordinary person, so brave, so talented. She made a huge impact on AGT and the world, her courage was incredible and her determination to fight this terrible illness was remarkable. Rest in peace, Jane. I am sending my love to her family. Simon."