Middlesex County is the most populated in Massachusetts and was established in 1643. When looking for Middlesex County public records, there is a good chance you will be able to extend your research back at least that far.
Modern records from 1921 until now are maintained by the Registry of Vital Records and Statistics for the state of Massachusetts. They're located in Dorchester and you can either visit in person to the service counter or send them a records request form by mail. They can issue any Middlesex County public records that you want copies of, because there are no privacy restrictions in Massachusetts.
The forms to make a vital records request can be picked up at the counter, but you can print them out home from the Office of Health and Human Services' website. There is one form for all types of vital records, so read through it so you are filling out the right fields for the documents you want.
After 1921, all records were moved to the State Archives facility in Boston. This collection of material then dates back until 1841 when the state first made vital record registration a county requirement. This material can be researched at the Archives in person, and many of their indexes are available online for remote searching.
And then beyond 1841, you have a third location to look for Middlesex County public records. Since they were recorded before the state requirement, they are still kept by the county clerk's office in Cambridge. They can look up and issue records just like the Vital Records office.
Middlesex County Birth Records
Middlesex County birth records will require you provide the full name of the person, their parents names and the date and location of birth. The fee for each record is $28 if you order by mail, but only $18 if you visit an issuing office in person.
Middlesex County Death Records
The fees associated with Middlesex County death records are the same as for birth records, and they are also issued to anyone who makes a request. You have to provide the deceased's full name, the location and date of death, and ideally also their parent's names.
Middlesex County Marriage Records
Getting a copy of a Middlesex County marriage record is no different from the others. They are issued at the same offices, and come with the same associated fees. The form will request the names of both parties, and the date and location where the marriage ceremony took place.
Middlesex County Criminal Records
Acquiring copies of any Middlesex County criminal records is a different process than for vital records. This information is not kept by the county, and you have to send your document requests to the Department of Criminal Justice Information Services (DCJIS). They are located in Chelsea but accept applications by mail. You can ask for a copy of your own records, but not all others are accessible. Whether a Middlesex County criminal record is considered public will depend on the nature of the crime, and what the sentencing requirements were. Each record ordered will cost $30, even for your own.