I inherited an IRA from an 83-year old (non-spouse) in 2017. I did not take the Required Minimum Distribution (RMD) in 2018 due to a mistake on my part. What can I do?

[dmertz]

Take the late RMD for 2018 now (and the RMD for 2019 if you have not already done so), then file 2018 Form 5329 Part IX requesting a waiver of the excess accumulation penalty.  Instructions are provided on page 8 of the Instructions for 2018 Form 5329 (see "Waiver of tax"):

https://www.irs.gov/pub/irs-prior/f5329--2018.pdf

https://www.irs.gov/pub/irs-prior/i5329--2018.pdf

The distribution is taxable on the tax return for the year the distribution is received (which should be 2019 if you act promptly).