Thanks, I am now looking into this myself. In my case the most recent ancestor born in Canada was my father's father's father, born 1884. My father is still alive, so he could potentially apply for the Citizenship Certificate, and for him it would just be his grandfather. Also my great grandfather was still alive in 1947 when Canadian citizenship was first defined, which may be an important link in the chain.
However, it seems (based on AI search) that in the early 1900s when my ancestors became US citizens, dual citizenship was not allowed by either the UK or the US. Becoming a US citizen involved verbally disavowing any allegiance to a foreign power, and British subjects were deemed to have automatically lost their citizenship in such a case. As I read it, to become a Canadian Citizen on 1 Jan 1947 when Canadian citizenship became a thing, my great grandfather would need to have been a British subject at the time, and it's likely that he'd already given that up more than 40 years before. The UK and Canada almost certainly had no record of this, since I doubt anyone notified them. AI also told me that at the time it would have been fully possible to maintain two passports and live as a dual citizen, despite the actual laws in both countries not allowing this. However, the actual law may still be relevant for deciding whether my family are now Canadian citizens.
On the other hand, I know that in modern cases, it sometimes happens that a minor can end up retaining multiple citizenship when an adult would not be able to, because they do not have the legal capacity to renounce their previous citizenship, and it's possible that my great grandfather was still a minor when the family got US citizenship.
It sounds like your Canadian ancestors are similarly old, do you have any more information about whether Canada has figured out how to treat these cases?
I learned a few weeks ago that I'm a Canadian citizen. This was pretty surprising to me, since I was born in the US to American parents, both of which had American parents. You don't normally suddenly become a citizen of another country! But with Bill C-3, anyone with any Canadian ancestry is now Canadian. [1]
In my case my mother's, mother's, father's mother's mother was Canadian. While that is really quite far back, there isn't a generational limit anymore.
Possibly you're also a Canadian citizen? Seems worth checking! With how much migration there has been between the US and Canada, and citizenship requiring only a single ancestor, this might mean ~5-10% of Americans are now additionally Canadian, which is kind of nuts.
I very much think of myself as an American, and am not interested in moving to Canada or even getting a passport. I am planning to apply for a Citizenship Certificate, though, since it seems better to have this fully documented. This means collecting the records to link each generation, including marital name changes, back to my thrice-great grandmother. It's been a fun project! I'm currently waiting to receive the Consular Report of Birth Abroad records for my mother and grandmother, since they were both born outside the US to American parents.
[1] This is slightly too strong. For example, it doesn't apply if you're born after 2025-12-15 (I'm guessing you weren't), and no one in the chain can have renounced their Canadian citizenship. But the caveats all exclude very few people.