Is there a way to mark all mail as read in Mail?

Select the first email and scroll down the list while holiding down the Shift key. Then from the Mail menu bar top of your screen click Message > Mark > As Read

You're welcome Benjamin 😎

i know this was over a year ago, but i want to also say thank you for this reply. I am new to Mail and don't know my way around it. I knew there must be some simple way to mark all messages as read. I was a Eudora user since 1995 and in Eudora, when i would get new mail, it would open whichever mailboxes had new mail and they would be right there, displayed on the desktop. This doesn't happen in Mail and i needed a way to know if i had new mail without having to open every mailbox and look inside it, but all the mailboxes had various amounts of unread mail accumulated in them, thousands in Gmail All Mail, and hundreds in the Gmail inbox, hundreds in FaceBook, etc. I searched these forums with the question 'how to mark all mail as read' but all the answers and threads i found addressed other more complex issues. I figured the answer to my question was too simple maybe to answer. It should be simple. I did find a good solution of creating a smart mailbox for new mail, but it combined all thenew mail instead of listing it by mailbox. And then, finally after two or three days of searching and not giving up, i just found your post. Now, all my mail messages, all thousands of them, are marked read and i can easily see which mailboxes have new mail in them. I really appreciate it, i don't like spending that much time trying to find out something that i think should be simple. Mail and Eudora are so different, the change is casualty of getting a new computer that has Mountain Lion.

A faster way than what has been suggested is using the cmd-A keyboard shortcut after selecting a single email. This will instantly select every email in the mailbox. After inputting the command, select "mark as read" from the right click menu.

Thanks you helped me so much!!