Does a mechanic lien expire?

Yes. But I think you are confusing two types of liens here, a mechanic's lien and a tax lien. Once a mechanic's lien is recorded, an action to foreclose on the mechanic's lien must be filed within 90 days. Technically, the mechanic's lien does not automatically go away, just the right to foreclose expires. In other words, a mechanic's lien stays on the county records as a "cloud" on the property title until the owner takes action to remove it. In comparison, a tax lien remains on the property until the property is sold or refinanced.

After you record a mechanic's lien, you must file a suit to foreclose on it no later than 90 days after the recording date. If you fail to do so, the mechanic's lien loses its legal effect. You then have an obligation to record a release of the mechanic's lien or be required to pay the homeowner's attorney's fees if they have to go to court to petition to have it removed. The government couldn't have recorded mechanic's liens -- most probably a property tax lien.

It does sound as if you have the nature of the liens confused. I recommend you consult a local attorney who can review the documents and provide informed guidance. (THESE COMMENTS ARE NOT LEGAL ADVICE. They are provided for informational purposes only. Actual legal advice can only be provided after consultation by an attorney licensed in your jurisdiction. Answering this question does not create an attorney-client relationship or otherwise require further consultation.)