Such fears, however, prove to be completely unfounded. There's still that unmistakable fiery red hair, the love of black eyeliner and those red lips that led to Shirley becoming the face of MAC makeup in 2002. But as the diminutive 40-year-old star strolls into the room in a vintage-looking gold and black striped shift dress and skyscraper heels, she is all smiles. Her only entourage is her mum Muriul, dad John and strikingly similar-looking sister Lindy.
"I'm really happy to be here," she grins, as she scans the Lord Provost's opulent private quarters within the City Chambers. "I'm a horrible, cynical whine so this is something special."
The Edinburgh born and bred singer, who grew up in Stockbridge, was awarded a civic reception on Tuesday evening for her musical achievements and her much-publicised love of the Capital which led her to be hailed as an "ambassador." And Shirley couldn't have been more proud.
"I was really surprised," she says. "This means more than a lot of awards that we've had as a band. I'm very proud of where I come from and I feel this plays a large part in my success, so to be honoured by the city I grew up in is a really big deal.
"It also means a lot to my family, especially my parents - they're beside themselves!"
A quick glance over to mum and dad reveals just that. Both Muriul, a former big band singer, and John are beaming with pride, posing for pictures and joking with Lord Provost Lesley Hinds. Shirley says that even her rock star friends, such as Gwen Stefani, were delighted to hear of her latest accolade. "Everybody that I know - my pals - those who are really jaded, are all really chuffed for me," she admits. "I feel rather unworthy, but hey."
Despite being able to entertain at sell-out shows around the world, hold her own in televised interviews and meet some of the world's biggest celebrities, Shirley is refreshingly modest for such a big star. And while she cringes slightly at being called an ambassador, it's clear she is just that.
"I've had a lot of international success and I have possibly travelled to every major city in the world, so to a certain degree it's always been in my head that I'm representing my country and, in particular, my city.
"It is a mind-blowingly beautiful city but we also have a liberal spirit at a time in the world where there's incredible strife. It's good to know that we Scots don't want to pick a fight with anyone - except each other!
"There's an incredible bonhomie. We have a wonderful characteristic of humour and joie de vivre, as well as our many cultural and social backdrops."
The interview turns to chit-chat - about her fond memories of Stockbridge, her love of the shops in St Stephen Street, the Modern Art Gallery, fish and chip shops with "that amazing brown sauce you can't get anywhere else in the world", not to mention her love of the Castle and the cobblestones.
Although she now lives in Los Angeles, she still sees Edinburgh as home. "Edinburgh is still my home, definitely. I have my own house here and I still pay my taxes here!" she jokes.
Often in life, we don't know what we have until it's gone, and we don't appreciate something - even our home - until it's no longer there. For Shirley, this is certainly true.
"I'd totally agree," she nods, resolutely. "It stunned me that the more I travelled the more I realised how Edinburgh can hold its head up high with all of these major cities.
"When I went to Paris, New York, Toyko I realised just how stunningly beautiful Edinburgh really is. I do miss it. And I do miss the Scottish sensibility a lot. Scottish people are just really funny and down to earth.
"A Scotsman is a friend instantly until proven otherwise. And that is really quite rare in the world these days.
"I've been on a variety of big shows abroad when they've had me on the couch and all they want to do is talk about Scotland. They don't want to talk about me or my career. And it's really lovely!"
Shirley comes home as often as possible, returning to her house in Joppa for extended breaks three times last year. She has been in the Capital for the past eight weeks, chilling out and catching up with friends and family.
"I was here for Christmas and New Year, the whole kit and caboodle, the whole [Hogmanay] cancellation! I ended up staying in with my sister and my mum and dad. It's my mum's birthday on Hogmanay so we always have a big family celebration."
Shirley is in good spirits. A year out to re-charge those batteries has worked. But her life in the Capital wasn't always so positive. Like many teenagers, she was bullied at school, which prompted her to self-harm. And those mental scars take years to fade.
"Fame doesn't make you more confident," she once said. "Maybe a lot of it stems from my childhood. Kids can be cruel and they were. They slagged me off, laughed about the way I looked and compared me to animals, bloodhounds, dead fishes."
To cope with the bullying, a teenage Shirley cut herself. She has said: "There is a sense of euphoria that follows cutting yourself. The quick pinch of pain and the sight of blood snaps you back to the surface and you start to appreciate being alive."
But Shirley is a fighter. She got over it. And to help prevent other teenagers go through her pain, as well as helping those who are going through it, she told her story in the documentary film CUT: Teens And Self Injury.
"It needs to be talked about," she says. "I think I started cutting because I was an inarticulate teenager which came out in strange ways, and the cutting was one of them."
So what's next for Shirley? And Garbage?
"I'm making a solo record in Los Angeles so I'm busying myself with that. And we've got the Garbage Greatest Hits coming out this year - but we're recording another song to go in it. We want to make it really good."
"I'm not putting any pressure on myself as I feel I've had that for ten years.
"I've taken a year off, I've had a life, I've actually had things in my refrigerator!"
And are the rumours of a Garbage split true? "Bands are like the mafia," is her carefully-worded answer. "Once you're in, you never really get out.
"I love them [the band]. We just needed a break from each other. We're actually playing a benefit show together next week as a friend of ours has had throat cancer and it's hard to have medical insurance over there."
So will she ever come back to Edinburgh for good?
"As I get older I think I'll want to come back, but right now I have a lot of business in America," she says. "My record company are based there so it makes sense to be near them - otherwise they'll forget all about me. The girl's gotta hustle, you know."
CITY GIRL WHO STRUCK GOLD WITH GARBAGE
SHIRLEY MANSON learned to play the piano at seven, and later attended the City of Edinburgh Music School.
Her career took off when she joined Goodbye Mr Mackenzie as a backing vocalist and keyboard player. The band became Angelfish, with Shirley taking lead vocals in 1993. Guitarist and Garbage founder Steve Marker saw Shirley perform on the video, Suffocate Me, on MTV. His offer of a position in his band was quickly accepted.
The self-titled album, Garbage, released in 1995, contained hits including Stupid Girl and Only Happy When It Rains. In 1999 Shirley became a Bond Girl of sorts, when the band recorded the title track for The World Is Not Enough.