Is it law or SOP that dual citizens can't become President/PM?

In Canada, there are no legal qualifications for the office of Prime Minister, as the office is not defined. So, in theory a non-Canadian could become Prime Minister. However, by constitutional convention, the PM must hold a seat in the Commons, and the Elections Act currently provides you must have Canadian citizenship to stand for election to the Commons.

British PM Winston Churchill’s mother was an American citizen.

True, but at that time citizenship/subject status was decided by the father’s status, so Winston was not an American.

Congress did grant him honourary US citizenship after the war.

True but inherited citizenship (jus sanguinis) is more difficult to establish. Being born in the United States gives a person citizenship automatically (jus soli). I mentioned that Johnson’s parents were in America for academic reasons because people who are serving their own country in America can be an exception to jus soli citizenship.

It’s amazing to me how many Americans believe this to be true. I’ve had people argue vehemently with me about this, even after I introduced them to people who hold both U.S. and non-U.S. passports.

My ex-brother-in-law has both U.S. & Japanese citizenship. Born in the U.S. to Japanese parents, he then bounced back and forth between Japan and the U.S. for most of his childhood. He had passports for each country from early childhood, and since he had a legal family registry in Japan (koseki), and was a U.S. citizen by birth, he was never asked to decide between the two (as a post above suggests).

Interestingly, his children are currently quadruple citizens. His wife is a U.S. - French dual citizen, and their children were born in New Zealand, and hence into NZ citizenship.

Not only that but under the law at the time his mother would’ve automatically lost her American citizenship by marrying a foreigner and acquiring his nationality. IIRC married women weren’t allowed a citizenship independent of their husbands until the 1920s.

Hitler renounced his Austrian citizenship in 1925, but didn’t get around to becoming a German citizen until 1932 to qualify for some job or the other.

The Austrians didn’t revoke his citizenship until 1927 — although the Anschluss probably made it all moot; but in the meanwhile he was officially stateless. AH was always ambivalent to Austria anyway, because he hated the Hapsburgs so very, very much.

As of the 1985 citizenship law, dual citizenship is generally lost. He might be grandfathered in. Those younger might be able to sneak by (looks like you have until 22 to choose).
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During the last presidential election in Ireland, there was a bit of controversy when it was revealed that one of the candidates, Dana Scallon, had become a naturalised US citizen some years earlier. The objections were largely based around the oath of renunciation. Scallon obviously was unaware that that oath is pretty much meaningless, and instead claimed that the US had a special arrangement with Ireland to allow dual citizenship. I don’t think it really harmed her campaign (she was never going to get more than a small percentage of the vote anyway, and her supporters are fringe uber-Catholics who probably wouldn’t vote for any of the godless heathens running against her), but who knows how it might have affected a candidate with broader general appeal.

The UK must be the most legally laid back country about this. Any Commonwealth Citizen with rights to residence in the UK plus any Irish citizen resident in the UK may vote in National elections. The only requirement of a Prime Minister is that he/she is answerable to Parliament (more recently the House of Commons) by membership thereof. Whether a legally settled Rwandan would be elected or chosen is another matter.

Have just discovered that Scotland is even more laid back- nothing to stop a European resident in Scotland from becoming an MSP. Meaning that citizens of almost a hundred countries could occupy the Scottish First Minister post.

Her lying about “a special arrangement with the United States” and Ireland didn’t bite her in the tush?

I am mulling over the idea that Boris could become UK PM and POTUS at the same time:)

On the other hand, you don’t have to be born here (and two of the last three PMs weren’t)

I believe all that’s required is that you renounce any other citizenships - they don’t have to accept your renunciation.

There is a not-unreasonable case to be made that our current Prime Minister is ineligible for his office for that very reason - he definitely had UK citizenship at one point, and has never publically shown any evidence of renunciation. This crops up in newspaper opinion pieces and blogs every so often, but I’m not aware of anyone trying to take it to an actual legal challenge

He gave up his US citizenship earlier this week. Partly to affirm his Britishness, but mostly to avoid US tax which last year hit him for capital gains made on the sale of a family property.

And even if he didn’t give up his US citizenship, he doesn’t meet the residency requirements for the presidency.

Right. If a Canadian wants to become a US citizen, the US official tells the Canadian to state formally to the US that they renounce their “Allegiance” to Canada, which is tantamount to renouncing citizenship, since citizenship implies allegiance, at least in law if not in practice. If you say it, the US will mark you down as having renounced your Canadian citizenship and will give you a US passport.

Canada then is like, “What are you guys talking about down there? If you really want to renounce your Canadian citizenship, you have to fill out this form here and file it at such-and-such office.”

The US is then like, “What are you talking about? This guy clearly renounced his Canadian citizenship, leave him alone!”

Both countries essentially “agree to disagree” about whether you are Canadian. Hence the situation where the US does not “recognize” your dual citizenship.

Two anecdotes. All my family are dual US/Canadian citizens. When one of my sons was a senior in college, he met with an NSA recruiter who told him that if he wanted to work for them he would have to renounce his Canadian citizenship. He didn’t.

The second story concerns the woman who, around 2000, became the president of Latvia. She taught at one of the Montreal universities, I forget which, and was a compromise choice when the new legislature couldn’t agree on a candidate. She had to renounce her Canadian citizenship in order to take up the position of president of the republic.

Incidentally, I have been asked by US border guards what my status is in Canada. Dual citizen, I answer and they nod. The clearly don’t object.

The “membership thereof” is a bit fuzzy too. The Queen could, in theory, ask me to try to form a government. If I could get the Commons to go along with our madcap scheme, then I’d be PM.

It’s a holdover from the old days when the entire British Empire shared the same nationality laws. Most Commonwealth countries used to grant local resident British subjects the same voting rights as local citizens, but have since stopped. IIRC New Zealand went the other direction and allows all permanent residents to vote regardless of citizenship (though only citizens can stand for election).

No it isn’t. Stop making shit up.