The 20-year-old is also the first female headliner at the festival since 2016, when Adele took to the stage.
The 'Bad Guy' singer first performed at the Somerset festival in 2019 on the Other Stage.
In 2019, when she first played at Glastonbury, Eilish was originally set to perform at the John Peel tent.
However, because of the success of her single 'Bad Guy', she was moved to the Other Stage.
After the set, Eilish thanked fans for 'not leaving' as a result of the multiple technical issues that occurred throughout her performance.
While on stage, she also reflected sadly on how she 'would love to go' to Glastonbury as an attendee rather than performer.
"This looks fun to go to," Eilish said.
THIS IS THE BEST PERFORMANCE OF BURY A FRIEND. THE FIRE. THE FIREWORKS. YESSSSSSS BILLIE #BillieEilish— ZAWENA B (@zawenaeb) June 24, 2022
Eilish's headline performance at Glastonbury today (24 June) marked the first full day of music.
The musician is also currently mid-way through a series of shows at the O2 Arena in London.
She is performing a total of six shows at the venue, all of which are sold out.
Of course, people were quick to react on Twitter with some calling the singer an 'absolute superstar'.
#Glastonbury2022 billie eilish is an absolute superstar wow! Love her so much what an incredible artist she truly is. So unique 🙂🥰🙌👏💕💯🔥— Rosie (@Rosie12_1994) June 24, 2022
The shows centres around her second album Happier Than Ever.
While Eilish has just become the youngest-ever solo headliner to perform at Glastonbury, the musician never thought she'd get to this stage.
In 2020, she told CBS News: "I don't want to be too dark but I genuinely didn't think I'd make it to 17."
Her second album sheds light on the mental health battles she's faced since being flung into the spotlight.
However, Eilish noted how she is feeling happier than ever since the period of depression she experienced when just starting out as a singer.
She said: "All I can say now is, for anybody who isn't doing well, it will get better. Have hope. I did this with fame riding on my shoulders.
"Now I love what I do and I'm me again. The good me. And I love fame… I love the eyes on me."
the anticipation i feel today for whether harry styles will join billie eilish at glastonbury is making me unhinged— Issy Aldridge (@issy_aldridge) June 24, 2022
While Eilish is Glastonbury's youngest-ever solo headliner, Paul McCartney – the festival's oldest-ever headliner – is set to take to the Pyramid Stage tomorrow.
Sunday afternoon will see the 'Legend' slot played by Diana Ross.
The festival will be closed by Kendrick Lamar later that day.
Rumours have also been circulating online as to whether Eilish could perform alongside Harry Styles.
Eilish recently told Apple Music 1 that she is 'constantly feeling like [she's] undeserving of everything' but that she thinks it's a 'good thing'.
She resolved: "But first of all, it makes you doubt everybody because you’re like: ‘Why would you choose me?’
"But with that being said, that’s why I’m even more excited about it and stoked to be the youngest headliner ever."
Featured Image Credit: BBC