Hi,

If I have non-immigrant visa for 90 days, how do I prolong it?

Do I need to leave Thailand?

I'm going to work in Thailand for 10 months (and stay up to 12 months), so I can´t really cross the border every 90 days.

What advice would you give me?

Thank you!

If you have a work permit (required) and your income meets the nationality requirements you visit immigration and submit paperwork for an extension of stay for one year from date of entry.

To expand a little on what Lop said.

If you extend for 12 months on the back of a work permit you don't need to do visa runs. Yo DO have to report your address to immigration every 90 days (can be done by post).

ALSO.

If you get the extension and then wish to leave the country for a short while you MUST obtain a re-entry permit from immigration (or at the airport) before leaving, otherwise your extension is cancelled and you have to start again from scratch (which is a right royal pain).