"I'd booked some dumb exercise class at eight o'clock and it was six - so my brother was like, 'Just get on the piano and see what happens.'"
Forty-five minutes later, the 21-year-old had written Finders Keepers, a song that's now firmly lodged in the Top 10.
"It's amazing," she tells the BBC. "I just wanted to make something fun for me and my friends."
Mabel has been making waves since 2015, when she released the slinky, sensual Know Me Better, with its seductive refrain: "I could go all day wearing nothing but your kiss".
But she's been around music all her life. Her parents are hip-hop legend Neneh Cherry and Massive Attack producer Cameron McVey.
BBC Music - Sounds Like Friday Night
Born in the mountains of Malaga, she was raised between Spain, London and Stockholm, learning piano at the age of five and choreographing routines to Destiny's Child with her sister.
She's no stranger to the recording studio either, accompanying McVey when he produced the Sugababes' debut album, One Touch, in 2000.
At the tender age of four, she managed to sleep through the whole thing. "Do you know what? It's still a problem!" she laughs.
"The vibrations of the bass make me so cosy. The other day I had a blanket in the studio and my brother was like, 'You need to move. You're not writing, you're napping!'"
The habit has earned her the nickname Lil' Bassy - and it's not just confined to the studio. "It's concerts as well!" she says. "If I put earplugs in, the muffled sound of a gig gets me.
"Not at my own shows though," she clarifies. "[There's] no sleeping if I'm on stage."
Given her background, Mabel's success might seem like a fait accompli. But for a long time, she avoided making music.
"I felt quite embarrassed by being my mum and dad's daughter," she once said. "I thought, 'People will never take me seriously.'"
She eventually overcame that fear and enrolled to study production and music theory in Stockholm. After graduating she moved to London.
There she was cast for a photo shoot in i-D magazine. That caught the attention of Skepta, who put her in his video for Shutdown.
Mabel's hip-hop tinged debut, Know Me Better, went viral soon afterwards, propelling Mabel onto the BBC's Sound of 2016 list.
'These things take time'
"These things take time," she says, noting that new artists need longer to nurture an audience in the slow-burn streaming era.
"It's more like America, where sometimes it takes years to break a record."
This is especially true of Finders Keepers, which first came out in March and later featured on Mabel's Bedroom EP - a 21st take on the '90s R&B of Brandy and Aaliyah.
Lyrically, the EP discusses control within relationships - "how one minute you can be in the driving seat, then that flips and you're very much out of control."
It's also about balancing out the male-dominated narrative of R&B - which is where Finders Keepers comes in.
"There's so many R&B songs where guys are talking about a clingy girl, like: 'I don't want a girlfriend and this girl's so clingy and blah blah blah.'
"But I'm a woman and I've been in situations that have been the reverse of that, so I wanted to tell that story."
Finders Keepers stands out even more because it's Mabel's first uptempo track. "I really struggled with it before," she says.
"I'm really good at the '90s slow jams. I've got that down. But I love to dance, so why wouldn't I make something I could dance to?"
The song's success, she says, "surpassed everyone's expectations and every other song I've ever done" - and it spurred her to write more in the same vein.
"I have like Finders Keepers fever now!" the singer says.
"Sometimes I go in the studio and I'm like, 'That worked so well, and I wrote it in 45 minutes so if I try wearing the same outfit and playing on the same piano it'll happen again.'
"But you know what? That's why I love music - because I'm such a control freak and it's the only thing that I can't really control.
"It's like magic."
Mabel's Bedroom EP and her Ivy To Roses mixtape are out now.