My mom wants to pay me for running all her errands and taking her to her appointments. OK to bill her for mileage? Fair?

Yes, it is lovely that Mom wishes to pay you for this. Mom should be careful in case she needs to get medicaid with 5 year lookback in future that she doesn't pay you in huge amounts, that is to say over a thousand or so, without stipulating she is paying you for a service and saving proof of this, so that she doesn't appear to be gifting. A simple diary would also work, a composition book without tear out pages and with no erasers (just cross out mistakes) and this would read something like "Paid Fy 25.00 an hour for three hours to accompany me safely to medical appointment, and (amount) for gas mileage."

In order to legitimize that you are hired in case she needs Medicaid and a 5 year look back. Make a contract and notorize. Keep good records and also report this as income in your taxes.

All responses here are ok. Depending on your specific situation we could offer more information.

IMO it would be a lot simpler to be paid for your time rather than having to keep track of mileage.

Keep very clear and accurate records so there is a paper trail. You will need to keep a mileage log, invoice her (itemized) and give her receipts for her payments. Your bookkeeping will matter.

The IRS mileage for 2023 is 65.5 cents per mile.