How to mark all unread mails as read in Gmail. Here’s a very quick tip to mark all the unread mails as read and reset the unread counter in Gmail.
Step #1. Type “in:all is:unread”, without quotes in Gmail Search box and hit Enter.
Step #2. Now select all the mails using “Select All” option.
Step #3. Now click on “Select all conversations that match this search”.
Step #4. And finally, click “Mark as read” in the “More” drop down menu.
Step #5. Done. This marks all your unread mails as read.