Do I report mileage reimbursement that wasn't included on my 1099?

Been there (gig work) and it's easy to accidentally double-count this.

If the mileage reimbursement wasn't included anywhere on the 1099/1099-NEC, I personally wouldn't go out of my way to add it as extra income just so I can deduct the same miles. The whole "report reimbursement then deduct mileage" thing only makes sense when the reimbursement was already baked into the gross they reported.

The part I'd focus on is: make sure you're not also taking the standard mileage deduction for miles that were actually reimbursed (depending on how you're treating the reimbursement). IRS basically doesn't let you get paid tax-free for the miles and still deduct the same miles again.

What saved me when I got my numbers straight was keeping a clean log of business miles (I use MyCarTracks now because I kept missing trips when I did it manually), and then matching that against whatever the payer actually reported. If you can reconcile those two, the reporting tends to fall into place.

If your situation is more like an employee 'accountable plan' reimbursement vs contractor, that changes the answer, so it's worth confirming what type of 1099 this is and how the payer described the reimbursement.

Only if you report the amount of reimbursement you received as income on Line 1 of your Schedule C. Usually there's no reason to add income just so you can expense the same amount later.