The Brisbane woman hopes to follow in the footsteps of former McDonald's Australia chief executive Catriona Noble, who also started flipping burgers at 14 and became the company's youngest ever female restaurant manager at 19.
Ms Noble moved to a regional role with the company in 2014 before heading to ANZ to run its retail mortgage business last year.
Her replacement at McDonald's, former chief financial officer Andrew Gregory, likewise started as a crew member in 1992, working his way through the ranks into management.
"Most people just think McDonald's is about flipping burgers for a living, but going through the ranks as a crew person, learning all the different areas, you begin to see there's a completely different side to it," Ms Bittner said. "You learn more about the business, to me now it's like a whole new world."
McDonald's employs around 106,000 Australians and is the largest youth employer in the country, with 65 per cent of its workforce under the age of 18.
Earlier this year, the fast-food company came under fire over a wage deal struck with the SDA union that allegedly left Australian staff worse off by $50 million a year. At the time, McDonald's said any inference that it underpaid its employees was "simply wrong".
McDonald's chief operating officer Shaun Ruming said the company's role as Australia's biggest youth employer was "a pretty big responsibility that we don't take lightly".
"With youth unemployment the way it is in this country, we feel we have a strong role to play in the economy," he said.
Ms Bittner and fellow Brisbane employee James Harrigan, also 21, are highlighted in a new advertising campaign focusing on the career opportunities for young people.
Mr Harrigan, who started working at McDonald's when he was 18, is now a department manager of customer experience and aims to become a licensee.
"Basically I've always wanted to work at McDonald's," he said. "Eventually I would love to become a restaurant manager, and my ultimate goal is to own one.
"There is incredible opportunity to keep going up through the ranks. Anyone who thinks they just come in and flip burgers and can't stay for a career is joking."
Mr Ruming said the company was averaging a crew turnover of 42 per cent per year, which was better than the overall retail industry. Management positions see less than 20 per cent turnover.
"We know when they become managers they can see the career mobility," he said. "A lot of the management and executive team are crew people who worked through the system - I started at 14 years and nine months [and I've been here] 30 years."
Mr Ruming said the company was trying to combat the stigma for new employees by overhauling its recruitment and orientation process to emphasise the job's transferable skills.
"About 84 per cent of people we talk to in the recruitment industry think it's a positive thing to have McDonald's on your resume," he said.
"Whether it's cash handling, food safety, customer service, management of employees, these are all transferable skills. I think for the last few years we have lost our voice on that."
The company is also trying to emphasise the flexibility it offers for young people. "These are busy young people with exciting lives and friends and Facebook," Mr Ruming said.
Ms Bittner, who has been competing in Gaelic dance since the age of five, said her bosses had been very supportive. "We work out with my availability so I can still make it to class, they make time for me to go overseas and dance each year for a couple of weeks," she said.
