Are McDonald fries halal in US?

yes, Of course!

So long it is not mentioned in the holy Quran or in prophet's Sunnah as Haram "Prohibited" such as ham, alcohols, blood ... etc... then it is Halal even if cooked by Christians or Jews. We can eat from their food and meant which they slaughter.

Improved answer:Well according to Islam, everything in this life is Halal so long it is not prohibited by a verse from the Holy Quran or by a Hadith from prophet Mohammed's Sunnah. This applies to all types of food and drinks too. Sea food is all Halal (if it does not kill you of course" even dead fish, Wales...etc. if still fresh they are Halal.

Allah said in the Quran:

ÙÅسألوÙâ€Ã™Æ’ ماذا أحل لهم Û– قل أحل لكم الطÙÅبات Û™ وما علمتم منالجوارح مكلبÙÅنتعلموÙâ€Ã™â€¡Ã™â€ مما علمكم اللـه Û– فكلوا مما أمسكنعلÙÅكم واذكروا اسم اللـه علÙÅÙ‡ Û– واتقوا اللـه Ûš إناللـه سرÙÅع الحساب ﴿٤﴾ سورة المائدة

They ask you, [O Muhammad], what has been made lawful for them. Say, "Lawful for you are [all] good foods and [game caught by] what you have trained of hunting animals which you train as Allah has taught you. So eat of what they catch for you, and mention the name of Allah upon it, and fear Allah." Indeed, Allah is swift in account. (4) Surat Al-Maida"

and prophet Mohammed peace upon him said:

قوله صلى الله علÙÅÙ‡ وسلم { ما أحل الله فهو حلال وما حرم فهو حرام وما سكت عÙâ€Ã™â€¡ فهو عفو ØŒ فاقبلوا منالله عافÙÅته فإنالله لم ÙÅكنلÙÅÙâ€Ã˜Â³Ã™â€° Ø´ÙÅئا }أخرجه البزار والطبراÙâ€Ã™Å منحدÙÅØ« أبيالدرداء بسÙâ€Ã˜Â¯ حسÙâ€

In prophet's hadith, the prophet said: What Allah has permitted is "Halal"", what Allah has prohibited is "Haram" and what Allah has skipped is pardoned "Pardoned". So accept what Allah has pardoned you from since Allah is not a forgetful. Narrated by Tabarani.

Exceptional for India:If the food is made by Pudists and Sikh, "is allowed" so long they did not slaughter the animal themselves and so long you trust them. But if they slaughter the animal, their food is no more Halal because they are not people of the book like Christians and Jews.

Improved above.

Read more: Is_It_any_Halal_foods_in_India