CBS News Philadelphia found that filling out your ballot using a Sharpie pen will not invalidate it, according to local county and state election officials.

Some posts say using a Sharpie on your ballot will invalidate your vote. But many ballot tabulator manufacturers actually recommend Sharpies over regular pens. ... St. Louis, MO » 86° St. Louis ...

Yes, you can use a Sharpie to fill out your ballot in some states. Many voting machine manufacturers recommend them over ink pens because they dry quickly and the machines read their ink better.