Denied entry to Canada. Will this history deny me entry to other countries?

Were you really “denied entry” to Canada? Check your paperwork and your passport carefully.

Often what Canada does for minor offenses is to allow you to “voluntarily withdraw” your application to enter Canada. You still get turned back at the border and a strike in your record on the Canada side but as you voluntarily withdrew your request to enter, it does not count as an entry refusal.

update: note that being given an “allowed to leave form” isn’t necessarily the same as a voluntary withdrawal. Some “allowed to leave” forms will specify that you were found inadmissible, give the section of the immigration act you violated or didn’t meet, and note that you are leaving voluntarily (ie not in handcuffs) -- in these cases, you were denied entry and should declare this when asked. Other "allowed to leave" forms do indicate that you voluntarily withdrew your application and there is no declaration of inadmissability. You need to check whatever paperwork you were given carefully.

For the OP, that you don’t have a denial stamp in your passport and that you were let into Canada a few days after strongly suggests voluntary withdrawal. But you need to double-check especially if you plan on traveling to other countries as visa-waivers often require no denials on your record. If you’re unsure and don’t have the paperwork, ask the CBSA agent next time you pass through or ask your local Canadian consulate.

Voluntary withdrawals are not entry-denials and so do not have to be listed when asked if you've ever been refused entry or had a visa denied. However, if you are explicitly asked whether you've ever voluntarily withdrawn an application to enter at the border, you do of course have to respond affirmatively.

As far as the weapons and drug charges, you again need to be certain of what happened: were you arrested? were you charged? did you plead guilty? were you sentenced or fined? or did you voluntarily give up the contraband in lieu of being charged/arrested/fined? These distinctions make a big difference to other countries when using visa waivers or making visa applications.

For example, in the specific case of Japan, they ask whether you have ever:

• been deported or removed from Japan or any country for overstaying your visa or violating any law or regulation?
• been convicted and sentenced for a drug offence in any country in violation of law concerning narcotics, marijuana, opium, stimulants or psychotropic substances?

Based on what you've said, it sounds like your answer to both is "no."

When you are asked whether you have ever been denied entry, the correct answer is Yes. I was turned back simply because I carried too much things in my car for my intended duration of trip. Nothing stamped in my passport, no implicated crime committed, no inadmissibility or anything like that -- just an "Allowed to Leave" form given, which says "I hereby voluntarily withdraw my application to enter Canada and agree to leave Canada without delay." Scroll forward a bit, when I subsequently applied for Permanent Residence, I had a question, "Have you ever been refused entry into Canada?", "Have you ever been refused entry into any other country?" I answered No to both, and lo and behold, they served me with a Procedural Fairness Letter, which comes before a Misrepresentation charge. I attach the image of the PDF of it, with my information whited out.

In the end, they gave me 7 days to provide them with additional documentation/explanation to allay their concerns, or else face the charge of misrepresentation. God has helped me, to where this refusal was recent enough, and my photographic memory worked well enough, that I could quote how the border officer explained this Allowed to Leave for me -- and also, they had record of the conversation in their system, to where they confirmed that the border agents messed up when explaining to me how I should fill this out on future applications; I also had records of the phone calls where I called the embassy to ask about this -- so after doing their investigation for almost 4 months, they decided not to raise the charge of misrepresentation. But the point is, if you are asked about being denied, the truthful answer would be Yes. So that's the bad news.

The good news is, that Japan does NOT share the same immigration system. The countries able to see this refusal are, US, Canada, UK, Australia, and New Zealand. Hence, unless they found out about this refusal in another way (e.g., the "Drug watch" flag might be visible to all countries; I don't know), the Japanese wouldn't know that you have been refused from entering Canada.