Using the US Mail to send your ballot is convenient. You can be anywhere in the country (or even the world) and send your ballot by mail.
But you need to send it early enough.
If you can't mail it at least a full week or more before Election Day, your best bet is to use a Ballot Drop Off Box or turn your envelope in at a Vote Center.
A mailed-in ballot has to be postmarked by 8:00pm on Election Day, and received by the Registrar of Voters within a week of Election Day.
Postmarks are not instant. They do not happen just because the envelope went into a mailbox.
The postmark is not necessarily even applied at the Post Office in the same ZIP code.
When mail is collected from postal boxes, it's usually grouped and sent to a Processing and Distribution Center (P&DC), where the postmark is applied. That may not happen the same night as the pickup.
For example: If the posted pickup time at your mailbox is 3:00pm weekdays, and you mail your ballot at 3:30pm on the Friday before Election Day, your ballot won’t even be picked up until 3:00pm on the following Monday. If anything goes wrong on the way to and through the P&DC, your envelope might not get postmarked until after the deadline on Election Tuesday, even though you mailed it four whole days in advance.
If it's getting late:
Find Nearest Ballot Drop Off Box
or
Find a Vote Center