At the age of 11, watching the news on television with her family at home in Dagenham, she saw the havoc created by a terrorist's bomb. With the idealism of youth she decided there and then to compose something that would change the world.
Twenty years on McFarlane, whose first album Until Tomorrow was nominated for a MOBO award shortly after its release in 2011 and marked her out as a rising star on the UK jazz scene, has more modest ambitions with her music.
"I try to write songs that affect people on an emotional level," she says. "Not everyone's good with words and so I see it as my role to express their feelings, or at least feelings that they can relate to, whether that's joy, sorrow, anger, pain, whatever, and if they can relate to what I'm singing then they'll feel involved. And that's the most important thing about being a singer - you want people to feel part of what you do."
She has never been shy about performing. At family parties she wasn't the sort who had to be coaxed from under table to do her turn. She sang and danced and having quickly gotten over her urge to save the world, she decided, at 14, that Stars in Their Eyes was her ideal passport to the music business.
It's an experience that she laughs about now but even as a young Lauren Hill wannabe she was aware that the crew were winding her up when they informed her during the heats that she was about to perform in front of millions of TV viewers. It was more like 400 people in a theatre in Manchester. She didn't make the finals and two years later she enrolled at the Brit school in Croydon with the intention of studying musical theatre rather than seeking fame and fortune.
"It wasn't really a fame academy when I went," she says. "I don't know what it's like now, although quite a few young stars have come through it since I left. I think I knew that Imogen Heath had been there early on and Amy Winehouse and Leona Lewis were there at the same time as me. It was the first time I felt I was being taken seriously as someone who wanted to do music.
"But it wasn't an easy way into the business. It would be quite a long day. I'd travel in from Dagenham and there would often be, certainly in the musical theatre department, evening classes and rehearsals. So you could be up at seven and not get home until close to midnight and that in itself was good training for what I do now. You need to develop discipline and stamina otherwise you'll get nothing done."
It was while she was at the Brit school that McFarlane discovered jazz. She had to do this for herself because her parents weren't big jazz fans but realising that many of the songs from the musical shows she was studying had been adopted into the jazz repertoire she began to listen to Sarah Vaughan for inspiration and Ella Fitzgerald for homework.
"I'm not a big fan of Ella's voice and her singing, although it's technically flawless, doesn't connect with me emotionally," she says. "But if you want to learn a song properly, she's the go-to singer."
After picking up gigs with prominent musicians on the London jazz scene, including Soweto Kinch and Gary Crosby's Jazz Jamaica, McFarlane decided to follow up her degree in popular performance from the University of West London by studying jazz improvisation, composing and arranging at the Guildhall School of Music and Drama.
An EP of acoustic jazz songs that she handed to the influential radio presenter and record company executive Gilles Peterson got her a deal with Peterson's Brownswood Recordings label and after her debut album's critical success and MOBO nomination, Peterson has encouraged her to write her own material and develop her own style. Her second album If You Knew Her has been equally warmly received and she's currently hard at work on another set of songs - or at least she should be.
"Sometimes I wait for inspiration," she says. "But I've realised that that's not enough. You've got to sit down and make yourself write and that's a good thing because I've found I really enjoy experimenting with different ways of writing and although sometimes I'll dash something off in half an hour, at other times it can be a long process. It's great when inspiration does strike, though - even if it means you have to sing the idea into your phone in some public place and get strange looks. Better that than the alternative because I know if I don't phone it in, the idea'll be gone by the time I get a chance to work on it."
Zara McFarlane plays The Blue Lamp, Aberdeen, on March 14 as part of Aberdeen Jazz Festival. www.aberdeenjazzfestival.com