If I did a backdoor roth conversion for a previous year, do I need to amend my previous year's return?

Yes, you will have to amend your 2019 tax return to create Form 8606 for the 2019 nondeductible traditional IRA contribution. Please see How do I amend a 2019 return in TurboTax?. Your 2019 tax liability will not change. 

 

You will do through these steps to enter the 2019 nondeductible contribution on your 2019 amended return (1040x):

1. Click on "Search" on the top right and type “IRA contributions”
2. Click on “Jump to IRA contributions"
3. Select “traditional IRA”
4. Answer “No” to “Is This a Repayment of a Retirement Distribution?”
5. Enter the amount you contributed
6. Answer “No” to the recharacterized question on the “Did You Change Your Mind?” screen
7. Answer the next questions until you get to “Any Nondeductible Contributions to Your IRA?” and select “Yes” if you had a nondeductible contributions before this tax year.
8. If you had a basis in the Traditional IRA before then enter the amount.
9. On the “Choose Not to Deduct IRA Contributions” screen choose “Yes, make part of my IRA contribution nondeductible” and enter the amount.

 

On your 2020 tax return you will enter the 1099-R for the conversion like this:

 

1. Login to your TurboTax Account 
2. Click on "Search" on the top right and type “1099-R”
3. Click on “Jump to 1099-R”
4. Click "Continue" and enter the information from your 1099-R
5. Answer questions until you get to “Tell us if you moved the money through a rollover or conversion” and choose “I converted some or all of it to a Roth IRA”
6. On the "Your 1099-R Entries" screen click "continue"
7. Answer "yes" to "Any nondeductible Contributions to your IRA?".
8. Answer the questions about the basis

 

You can verify your entry by looking at line 4 of Form 1040:

1. Click on "Tax Tools" in the left menu
2. Click "Tools"
3. Click "View Tax Summary" in the Tool Center window
4. Click on "Preview my 1040" on the left