The scheme refused to pay Brewster the pension based on Local Government Pension Scheme (Benefits, Membership and Contributions) Regulations 2009, which said unmarried co-habiting partners should be nominated by their scheme member partner to be eligible for a survivor's pension.
But convinced she had a just case, Denise launched a fundraising campaign and took her case all the way to the Supreme Court.
Ms Brewster and Lenny McMullan lived together for 10 years and owned their own home.
Brewster lived with McMullan for around a decade before he died two days after they became engaged on Christmas Eve 2009. The High Court in Belfast originally ruled in her favour, but this was overturned on appeal.
"With more than six million people living together as couples and the numbers rising every year, this is an issue that needs to be addressed as a matter of urgency". She argued that she was the victim of "serious discrimination" and has now won her case at the Supreme Court.
The case could have implications for all pension schemes.
Today's ruling could mean that other public sector schemes could change their rules so unmarried couples automatically benefit from survivor's pensions without being opted in.
Under the regulations, married partners automatically obtain a survivor's pension, but unmarried partners only receive a pension if there has been compliance with an opt-in requirement.More news: Apple Inc. (AAPL): Apple Plans To Setup Manufacturing Plant In India
Five justices at the UK's highest court have now backed Ms Brewster's right to receive the allowance.
He said: "It's UK-wide because it's the Supreme Court and, without being too legalistic and precise about it, it's sent a very strong signal to any pension scheme that has different processes, different hurdles that co-habiting couples have to jump through that they would be very vulnerable to legal challenge".
The relevant clauses of European Convention on Human Rights lay down that you must have peaceful enjoyment of your possessions, and that your rights should be secured without discrimination. In addition, her deceased partner had not completed the nomination forms that would have designated her as the recipient of his occupational pension in the event of his death.
The Court says the 2009 Regulations already require a surviving partner to prove that a genuine and subsisting relationship existed, so the nomination requirement adds no extra value.
The rule which the Supreme Court has declared was unlawful is found in most of Britain's public sector pension schemes, of which there are around 12 million members.
He said: "It is also found in many defined benefit pension schemes in the private sector, of which there are around 11 million members".
"While the Human Rights Act does not bite on private sector schemes, members of those schemes will expect their pension scheme providers to follow suit and to operate schemes which do not discriminate unfairly on grounds of marital status".