The former England Under-19 international made four appearances last season as the Belles were promoted from Women's Super League Two.
"I have tried so hard to get back but my legs just won't go," she said in a message posted on Twitter.
"Unfortunately, this disease cannot be beaten and has taken my dream."
Belles manager Glen Harris said: "It is sad that a young player, with the talent Ash has, has had to stop playing at the top level.
"Ash has been a pleasure to work with, always turning up and training with a smile on her face... even if it's not always in the right kit!"
Mills added: "My three years with the club have been absolutely fantastic.
"I have enjoyed every minute of my time here. I feel very honoured to have represented this great historic and successful club."
Mills said she plans to cheer on the Belles from the stands.
Doncaster start the new Women's Super League One season at home to reigning champions Chelsea on 24 March.
Around 100,000 people in the United Kingdom have multiple sclerosis or MS, according to the Multiple Sclerosis Society.
It is normally diagnosed in people between the ages of 20 and 40.
Symptoms might include fatigue, vision problems and difficulties with walking, but MS is different for everyone.
In MS, the protective layer surrounding nerve fibres in the brain and spinal cord - known as myelin - becomes damaged. The immune system mistakenly attacks the myelin, causing scarring or sclerosis.
The damaged myelin disrupts the nerve signals - rather like the short circuit caused by a frayed electrical cable.
If the process of inflammation and scarring is not treated, then eventually the condition can cause permanent neurodegeneration.
Michelle Mitchell, chief executive of the MS Society, said: "MS is an unpredictable and challenging condition to live with and diagnosis can be upsetting and scary. The MS Society is here to help anyone affected by MS."
MS Society helpline: 0808 800 8000.

Mr Justice Mostyn said the game could be said to be  a legitimate sport under English law.
He has granted the Aylesbury-based English Bridge Union permission for a full judicial review of its status.
Sport England has refused to recognise the game and said it was no more a sporting activity than "sitting at home, reading a book".
Mr Justice Mostyn, who said he played Bridge on social occasions, heard a claim the game ought to be recognised as a "mind sport" that exercises the "brain muscle".
Kate Gallofent QC, for Sport England, said the definition of a qualifying sport was an "activity aimed at improving physical fitness and well being, forming social relations and gaining results in competition".
That definition is based on a European Sports Charter promoting "a common European definition of sport".
However, the judge was told other EU countries, including the Netherlands, Ireland and Poland, "recognise Bridge as a sport".
Miss Gallofent insisted: "The starting point of the definition of sport is physical activity. Bridge cannot ever satisfy this definition."
Mr Justice Mostyn told her: "If the brain is a muscle, it does.
"You are doing more physical activity playing Bridge, with all that dealing and playing, than in rifle shooting.
"In 1999, the International Olympic Committee (IOC) recognised that Bridge and Chess should be recognised as mind sports."
The judge granted the union permission to mount a full judicial review challenge to Sport England.
Contract Bridge is a complex game played by millions of people across the world in competitions and at social events.
All the cards are dealt from a normal 52 card deck for each game.
Two players from each of two teams sit opposite each other across a table and through a bidding process (auction) agree to win a number of tricks.
Suits are ranked upwards - clubs, diamonds, hearts, spades and no trumps.
If they achieve the number of tricks agreed at auction they win the game but if not they lose.
The team with the highest number of games takes the rubber.
These are the simple basic rules. At higher levels, the game includes bonus points, complex dealing and other laws.
Doncaster Rovers Belles midfielder Ashleigh Mills has ended her playing career at the age of 20 after being diagnosed with multiple sclerosis.
[NEXT_CONCEPT]
The card game Bridge is "arguably" a sport and could qualify for lottery funding, a High Court judge has found.