How do I add a transaction sub-category?

From Mint.com:

  1. Sign in to Mint.com. You can't do this in the app.
  2. Select Transactions near the top of the page.
  3. Choose the transaction you’d like to update and select its existing category.
  4. Select the arrows that appear next to the category name.
  5. Select Add/Edit Categories…
  6. Select the corresponding main category from the list and select ADD A CATEGORY.
  7. Enter the name and select SAVE.

If you want Mint to use this category for all future transactions from this merchant, you can create a rule.