I have been living in Mexico for 15 years. If I return to the US will I be eligible for coverage?

I think the issue turns on whether or not you relinquished your citizenship or remained a US citizen. What kind of health coverage did you have in Mexico?

What coverage are you referring? Do you mean Medicare? If you are a U.S. citizen and say you worked with a U.S. company that had offices in Mexico, hopefully the company took out payroll taxes to pay toward Medicare. If not, you would need to check with Medicare itself to see if you would be eligible.