For Heath, who died when he was just 28 from acute drug intoxication, becoming a father was a turning point in his life.
The Dark Knight star admitted in 2006 that family "is everything to me – the most important thing I do."
"Matilda is adorable, and beautifully observant and wise. Michelle and I love her so much. Becoming a father exceeds all my expectations. It's the most remarkable experience I've ever had—it's marvelous," Heath told Intouch in December 2005, when his daughter was six weeks old.
Michelle and Heath fell in love on the set of the critically acclaimed 2005 drama Brokeback Mountain, but split in September 2007 - just four months before his death.
When Matilda was a baby, the family-of-three were frequently spotted in their Brooklyn neighbourhood.
But little is known about Matilda, who is now 17 and the spitting image of her superstar parents.
Since Heath's passing, Michelle has managed to raise Matilda almost entirely out of the public eye.
The Shutter Island actress took a hiatus from the spotlight and moved with Matilda out of New York City in the wake of Heath's death.
The mother-daughter duo moved to a farmhouse in upstate New York where she sought a “a sane home environment” away from the limelight.
"We moved—we left Brooklyn and lived in the country for six years because it was an intolerable state of existence," she told the New York Post in June 2016, referring to the barrage of paparazzi following Heath's death.
Matilda remains largely mysterious due to her notoriously private mother opting to keep a tight lid on her own private life.
Michelle, now 42, also avoids social media to keep her daughter off the radar of the prying press.
In 2015, Heath's sister Kate Ledger said Michelle "keeps things real" for Matilda, who was 10 at the time.
"And her existence, although different from most, is as normal as possible in an abnormal world," she told the UK's Sunday Telegraph.
Heath's father Kim Ledger gushed that his granddaughter had his son's infectious energy in her early years.
"Heath never slept from when he was two and Matilda's like that. She's just got this ball of energy and she radiates this little aura," he said.
Heath's mother Sally added: "Matilda is indeed very much like her dad - she has long limbs and similar features. However, she also possesses the beauty of her mamma - lucky girl."
Though Matilda was just two years old when her father died, Michelle admitted she sees Heath in her.
"Every time I really miss him and wonder where he’s gone, I just look at her," the Annette actress said in 2009.
Michelle has also made sure that she keeps the memory of Matilda's late father alive.
“I always say to Matilda, ‘Your dad loved me before anybody thought I was talented, or pretty, or had nice clothes,'” she told Vanity Fair in 2018.
"When you’re a single parent, and that element of provider and protection is missing, it’s scary," she said of raising Matilda after Heath's death.
Matilda is even carrying on Heath's legacy by taking up an interest in chess - a game the Ten Things I Hate About You star was enamoured by.
"Heath was obsessed with chess, and now Matilda is getting quite good at it,” a source told WHO this year. “She even has her dad’s old chess board."
In 2012, Michelle opened up to GQ about the prospect of having more children and giving Matilda siblings.
"I really wanted, and I really expected or imagined, that Matilda would have siblings that were close to her age. I wanted that for her. But I couldn't make that happen," she said.
"And now that she's 6 that isn't even a possibility anymore. So something that was making me feel impatient, that's been removed. For whatever reason, that's not our luck, or our path."
The My Week With Marilyn star has since moved on and found her happy ending with theatre director Thomas Kail.
In 2020 the couple tied the knot and welcomed a baby boy, giving only child Matilda a long-awaited little brother.
"[Michelle] is very excited to have another baby and give Matilda a sibling," a source told E! News following the pregnancy announcement.
In 2022, the couple have welcomed a third child.
While details are yet to be known, during her pregnancy, Williams told Variety, “It’s totally joyous...As the years go on, you sort of wonder what they might hold for you or not hold for you. It’s exciting to discover that something you want again and again, is available one more time."
As a (now) older sister to two, Matilda maintains her privacy.
In a rare image, her striking resemblance to her father is immediately apparent.
The family-of-five now live a relatively quiet life in the Brooklyn neighbourhood of Red Hook.
