The Havant-based athlete was inspired by watching David Weir win four golds at the London Paralympics a decade ago.
Now her coach, Weir was on the finish line to greet his emotional charge.
Australia's Madison de Rozario, one of wheelchair racing's top international stars, took gold in a new Games record. But Rainbow-Cooper was just under four minutes behind, cracking the two-hour barrier for the first time with a 1:59.45 personal best.
‘Everything is racing through my mind, it's hard to make any sense of what I'm thinking right now, I'm just so happy,’ she said.
‘I was just hoping for a personal best but I never in a million years thought I'd win a medal; I was just hoping to enjoy it.
‘This is only my second marathon and to be in such an elite field is so inspirational.’
Back in May, Rainbow-Cooper won the Schnneider Electric Marathon De Paris for Disabled Participants – the first marathon she’d ever done.
Rainbow-Cooper, 21, admitted the twisting turning circuit around Birmingham, which included a series of punishing climbs, pushed her to the limit.
She believes she needs another two years to truly understand the unique requirements of the discipline, which could be the perfect build-up to the Paris Paralympics just over two years away.
‘That course was insane, the gradient on those hills at the end, that really tested me to the limit, I've learned so much in this race,’ she added.
‘I know I need another ten marathons to really get this discipline. I've got a long time to learn it but I'm so excited to get back to working and building on this.’
Rainbow-Cooper was born with a condition called Sacral Agenesis, a neurological disorder which affects her body below the waist.
She loved PE at school, but it was watching Weir in 2012 that changed her life, enrolling at his academy in south-west London and working with his coach Jenny Archer.
Weir was there to watch her cross the line, despite his own disappointment. He was looking at guaranteed gold in the men's race before a puncture just six miles from the finish line.
‘Being part of this team is really special,’ Rainbow-Cooper added.
‘I've known David Weir since I was a teenager, he's made such a special contribution to my career. We're so close, he's never wavered in our belief in him.
‘I'm so disappointed for him but it says everything he was there to say well done to me - he's a legend of sport.’
Weir paid tribute to his young charge, despite his own upset.
‘I first met her when she was 13, I knew pretty much straight away she was going to be an amazing athlete,’ he said.
‘She works so hard and is so committed. You have to hope this is just the start for her, she's got lots more Commonwealth Games and Paralympics to come but she's got her first medal.’
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