We take thermometers for granted and there’s no reason why we shouldn’t, they’ve been around since the late 1500’s.
It’s even possible that ancient civilizations had their own devices to measure how hot it was. But on record, the first person we associate the thermometer with is Galileo Galilei who came up with one in 1596. If you want to get technical, Galilei didn’t quite invent an actual thermometer, which measures the difference in temperature, he invented a thermoSCOPE, which only indicates a temperature difference.
After Galilei, we had to wait until 1612 for Santorio Santorio (yes, the same first and last name!) to invent the actual thermometer. It wasn’t a very good thermometer, but he still gets credit. Then the Grand Duke of Tuscany, Ferdinand II, showed up in 1654 and created a sealed thermometer with liquid inside which worked much better than Santorio’s creation, but was still very inaccurate. It took Mr. Gabriel Fahrenheit in 1714 to come up with the idea of using mercury as the inner liquid for the thermometer.
Mercury was much more reliable and had a predictable expansion rate so a better measuring scale was soon to follow. One of the first scales was brought about by Fahrenheit himself and is, of course, still used today.