Do I need a visa or a visa waiver ? I am transiting through USA to visit Mexico. If I have a Canadian passport, but I'm also a dual citizen of Iran, do I need a visa just for the transit ?
Also, I am living and working in UK with a professional work visa. So my trip will start from UK, and transit from USA and into Mexico.
I presume you will be traveling on your Canadian passport.
You do not need a visa. There have been recent changes to the visa waiver program affecting, among others, people who hold Iranian citizenship, but Canadians do not travel under the visa waiver program. A Canadian citizen's visa-free travel to the United States is governed by different provisions of the law.
See, for example, this list of visa waiver program countries, from which Canada is absent: https://travel.state.gov/content/visas/en/visit/visa-waiver-program.html
