Action 4 Equality Scotland campaigner Mark Irvine wanted to know if South Lanarkshire Council had paid more money for jobs traditionally done by men, and had asked the council for details about its pay scales in May last year.
The council refused claiming disclosure would breach the Data Protection Act, and ultimately took the Scottish Information Commissioner to the Supreme Court in London after the Commissioner said Mr Irvine had a right to the information.
The council also accused the Commissioner of "a breach of natural justice" because he did not copy its officials into all of his correspondence with Mr Irvine and two MSPs who were following the case during the course of his investigations.
Supreme Court deputy president Lady Hale today dismissed the council's arguments, and ruled that the Commissioner was right to back Mr Irvine's request for information.
The ruling states: "The Supreme Court unanimously dismisses the council's appeal.
"It holds that the Commissioner was entitled to reach his conclusion that disclosure of the information should be given by the council to Mr Irvine, and that there had been no breach of the rules of natural justice when the Commissioner did not copy the correspondence to the council."
The cost of the entire legal action is approximately £100,000, according to South Lanarkshire Council.
The Commissioner has spent £67,836 defending its ruling to date, although its costs have still to be finalised, bringing the total cost to the public purse so far to around £168,000.
The council said it is "disappointed" by the Supreme Court's decision but will now release the information "as soon as is practical".
The council had originally dismissed Mr Irvine's request as "vexatious", a legal term meaning burdensome, frivolous, annoying, disruptive, harassing, unreasonable or disproportionate, according to Commissioner guidance.
This was "principally because of Mr Irvine's blog for Action4Equality Scotland and his connections with the solicitor representing equal pay claimants against the council", the Supreme Court said.
The council later withdrew this claim and substituted it with a refusal on the grounds that it would breach the Data Protection Act (DPA), which protects the confidentiality of personal data.
It argued that "Mr Irvine had no legitimate interest in disclosure of the information and that disclosure was not necessary for the purpose of his legitimate interests".
However, the Commissioner ruled Mr Irvine did have a legitimate interest "given the considerable sums of public money involved and the fundamental issues of fair and equal treatment".
Furthermore, he could find no grounds to support the refusal on the grounds of DPA and subsequently asked for the information to be released.
The Supreme Court backed this decision today, stating: "As the processing requested would not enable Mr Irvine or anyone else to discover the identity of the data subjects, it is quite difficult to see why there is any interference with their right to respect for their private lives."
Current Information Commissioner Rosemary Agnew, who took over the post from Kevin Dunion last year, said: "I am pleased that the ruling by the Supreme Court supports our own carefully considered conclusions on this case.
"While the relationship between FOI and data protection law can be complex, the Supreme Court's ruling confirms the robustness of our approach and is a clear guide to how similar cases should be handled by Scottish public authorities.
"Importantly, the ruling also means that the requester can, at long last, receive the information to which he is entitled."
In a statement on the Action4Equality website, Mr Irvine said: "After three long years, justice and common sense have finally prevailed - although no thanks to South Lanarkshire Council which has wasted over £200,000 of public money fighting this case.
"The council originally dismissed my request as 'vexatious' but now it is one of Scotland's largest council's which is left looking rather silly and, after this decision, with egg all over its face.
"The council and its political leadership have lots of questions to answer over this debacle and the terrible waste of public money involved.
"The wheels of justice turn very slowly at times, alas, but today they finally caught up with South Lanarkshire Council which should hang its head in shame."
Council leader Eddie McAvoy said: "I am very disappointed at this outcome, and all the more so because we were told repeatedly by our legal advisers that our case was sound and that there were good grounds for the council's arguments.
"Given the judges' ruling, I have instructed officers to release this information as soon as is practical."
A council spokeswoman said: "This judgment is disappointing. We have never denied this information to people with a legitimate interest in it, where it is necessary and warranted, and that's why it has been provided to those representing employees and ex-employees in tribunals.
"However, we have been acting on legal advice that, if we released it to a third party, we could be in breach of the Data Protection Act by disclosing information which could be used to identify specific people and their salaries.
"Given that concern and our legal advice, it was clear that these arguments had to be heard in the highest court in the land. Indeed, the judges make it clear that they took our arguments seriously and believed our case was worth putting before them."