Now the former NFL defensive lineman can finally exhale as Leah celebrates the crucial five-year mark of being cancer-free on Wednesday.
"Once your child reaches the five-year mark in their cancer battle, the chances of the cancer coming back are basically slim to none,'' Still told TODAY. "This is huge for us because when you have a child that's battling cancer you're basically holding your breath until you reach this point."
The celebration has been subdued given that the family is following state and federal guidelines and quarantining themselves at their Houston home in the midst of the coronavirus pandemic. Leah told TODAY she's celebrating "no more hospitals and needles."
She just had a few special requests to mark a day that seemed like a long shot when her cancer journey began as a 4-year-old.
"It's great," Leah told TODAY. "We're gonna eat cake and ice cream and pull an all-nighter and watch movies."
"A day like this seemed so far away when we first got that diagnosis," her father said. "There's been a lot of ups and downs along the journey and times you question if you're ever gonna make it to this point, so it's just a blessing."
Still also marked the day with a sweet Instagram post about his daughter.
"Leah you are the strongest person I know,'' he wrote. "I thank you from the bottom of my heart for giving this battle everything you had. I know it wasn’t easy...I know it wasn’t fair...but you handled this battle like a #playmaker and inspired millions (especially me) while doing it. This is just the beginning I love you with every bone in my body."
Leah became a public face of pediatric cancer when she was diagnosed with stage 4 neuroblastoma in 2014 and given a 50-50 chance to survive while her father was trying to earn a spot on the Cincinnati Bengals.
She helped raise more than a million dollars for cancer research, won an ESPY Award for her courage, became good buddies with TODAY's Hoda Kotb, and inspired TODAY's "Truly Brave" music video, which starred Cyndi Lauper and Sara Bareilles.
While they enjoy a special day for Leah on Wednesday, the Stills also know that many other families are still in the midst of a cancer journey or have lost a child before he or she reached the five-year mark.
"There's a lot of survivor's remorse because you see the pain in these family's faces," Still said. "A lot of them look at Leah and be like why didn't their kids survive, and I can understand.
"Although Leah's cancer fight is over with, we gotta fight until every parent gets to feel this feeling of reaching five years."
When Leah was first diagnosed, the Bengals kept Devon on the practice squad on purpose so that he would have health benefits to pay for her treatment. He knows other families aren't as lucky when their child gets diagnosed.
That's why Still and his wife, Asha, both 30, started the Still Strong Foundation to help families facing difficult situations as their child is treated for cancer, which he feels is more important than ever given the specter of the coronavirus.
"A lot of people don't know COVID-19 is impacting kids battling cancer," Still said. "If they catch this virus, they can die. There's a lot of families struggling right now with not being able to go to work due to the virus and dealing with other issues, and we just want to be able to help them out."
While the foundation had to postpone its planned fundraising gala on March 28 due to the coronavirus, it launched the 5 for 5 campaign Wednesday to raise money to help families with things like medical bills or job loss.
The campaign asks for people to celebrate five years of remission for Leah by donating $5 and tagging five friends to also donate to help families in need.
Still retired from the NFL in 2017 and now runs the foundation full time and does public speaking about resilience and leadership at corporate events. He is grateful to see moments in Leah's life that he thought he might never witness.
"There's a lot of milestones that she has reached," he said. "She played soccer for the first time last year, and I watched her score her first goal. Now I understand that feeling my parents had going to my games.
"Getting to go to daddy-daughter dances for Valentine's Day, those are the things I could've missed out on if we didn't make it through this storm."
Leah, who enjoys soccer, swimming and making TikTok videos, is also now a big sister to 6-month-old Aria.
"I prayed to have a girl," Leah said. "Now Daddy wants a boy so I'm gonna pray for that because it works for me."
Still joked that Leah is now "9 going on 21."
"She's way more mature,'' he said. "She's not that same little girl I remember. She's grown so much."
On her special day, Leah had a simple message for other children and families hoping their child reaches the five-year milestone she is celebrating.
"You're not alone,'' she said. "Be strong."