Steps to completely delete your Tumblr account and all associated blogs you currently run on the Tumblr network.

1. Delete Tumblr
Account
WWW.HOWTODELETE.ORG

2. Things to consider before deleting
Tumblr account
 All the data in your primary blog will be removed
forever, and, this can’t be retrieved later.
 If you have a secondary blog where you are the
only member, you will lose that too. You will be
removed from any group blog where you are a
member.

3.  Your blog URL will be released. You can use the same
URL anytime when you wish to come back, but the
URL might have already been taken by somebody
else.
 Your Tumblr account will no longer exist, and people
will not be able to search for you on Tumblr or have
access to your profile anymore.

4. Delete Tumblr Account Permanently
 Visit Tumblr.com in any
browser you prefer either
from PC or from mobile.

5.  Log in to Tumblr with the email address and
password of the account that you want to delete if
you are not already logged in.

6.  Once you are logged in, click
on the profile icon that lies
towards the top right corner of
the page.

7.  Click on the Settings option in the drop-down menu.
 In the settings page scroll down to the bottom of the
page and then click on Delete Account.

8.  Now you are asked to enter your
login email and password to
confirm the deleting account.
 Finally, click on Delete Everything.

9. Deleting Secondary Blogs/Group
Blogs using Web Browser
 Go to Tumblr.com in any web browser and log in with
the username and password of the account if you are
not already logged in.
 Once logged in click on the profile icon and then select
the “Settings” option from the drop-down menu.
 In the right panel of the page, click on the name of the
blog that you want to delete or group that you want to
leave.
 Finally, scroll down to the bottom of the page and then
click on “Delete Blog” “Leave Blog”.

10. Deleting Secondary Blogs/Group
Blogs using Android Device
 Go to the Tumblr mobile application on
your Android device.

11.  Tap on profile icon (Person like
structure) at the right bottom of
the screen.

12.  When the page loads, tap on setting
option at the top right corner. And tap
on ‘Delete this Tumblr’.

13.  Now a pop-up box will appear
asking your password to
confirm the deletion of your
Tumblr account. Tap on ‘Delete’
to terminate your Tumblr
account.

14. Delete Tumblr Group Blogs using iOS
Device
 Open the Tumblr app on your iPhone or iPad.
 Tap on the profile icon towards the upper left corner of the home screen
and then tap on the blog’s name in the top left corner of the page.
 Find the blog you want to delete and then swipe it left and then choose
“Delete” or “Leave.”
If you ever think of returning back to Tumblr, you can always create a new
Tumblr account with the same username provided that any other user does
not already take the same username or URL. But keep in mind that no data
of your old profile will be carried forward to the new account even if you use
the same URL and credentials.

15. Source:
https://howtodelete.org/delete-tumblr-account/