Tax Year Prior to 2020: How do I handle a when a client paid me in 2016 for 2017 services? Note* this was reported on a 1099.

Assuming you use the Cash Method of accounting (as opposed to the Accrual Method), you will report it on your 2016 tax return.  The Cash Method is most commonly used and it simply means that you report income and expenses when received/paid.

Also, since you received the Form 1099-MISC, the IRS will expect to see it on your 2016 tax return.

[edited 4/8/2017]