It is absolutely true in this case that legalizing something does not mean people convicted previously are automatically pardoned. Canada has a process in place to allow those with previous convictions for "simple possession" to apply for a pardon, but it has not yet happened. You should also be aware that importing marijuana into Canada remains illegal, so your husband would still be guilty and banned if he did this today.

@DJClayworth Does OP say that her husband was caught importing marijuana? Maybe he was caught with it by the police inside Canada?

She needs also to recognize that immigration issues are federal issues both in Canada and the USA and hence even though some states have overturned the ban and decriminalized possession for personal use, the federal position remains unchanged.

+1 for "This issue is not one for soliciting answers from random strangers" - but answering nonetheless - we know our limits.

@Honorary The American federal position remains unchanged, which might not matter when you're tying to get into Canada

If the penalty for importing marijuana has changed, they may have some ground to stand on.

@Rogem the trickiest part is it is not automatic. Don't assume everything is fine just because the past offense is no longer illegal. A competent lawyer needs to work through it and officially engage the judicial system to clear the record.

"This is not the kind of question for which you solicit answers from random guys on the internet." comes off unnecessarily condescending. It looks to me like what actually happened here is the OP asked a good question, to non-random guys on the internet, and it had excellent results: that is, you replied with some excellent advice, apart from the odd tone. Right? So maybe clean up that part?

@Rogem Why? The offence was committed in the past. The punishment was given in the past. That it would not be an offence today, or that it would carry a different punishment today, is irrelevant, because it hasn't happened today.

@DonHatch No offense meant. I think you perhaps are the only one offended by that statement. The statement about seeking advice from random guys on the internet is a standard statement on travel.stackexchange used by multiple posters of repute to emphasize that professional advice in strongly recommended.

@LightnessRacesinOrbit It's not unheard of for criminals to be pardoned from sentences, after the legal system has moved to more lenient forms of punishment for that crime. Especially since the punishment wasn't in the past, but rather is still ongoing. He also has family ties to Canada that he didn't previously, meaning that the purpose of his visit is less likely to be intent to commit a crime, all of which is something for a legal professional to weigh, and not us strangers on the 'net who aren't even privy to all of the facts.

@Rogem All your points are true, although the OP doesn't say whether the family ties are new.

@LightnessRacesinOrbit It's relevant because the underlying moral code also changes. Replace "possession" with "homosexuality" if it's not clear: a previous crime was made legal that (according to some) never should have been a crime in the first place.

@KonradRudolph Drug users are not a protected group

@LightnessRacesinOrbit Nor did I claim they were. That’s utterly irrelevant to the larger point that moral codes, which underly and guide penal codes, change. And this is patently the case for drug possession, even if not as obviously as for sexuality.

@KonradRudolph You used homosexuality as an analogous example as if it can be supplanted directly into the actual scenario being discussed without introducing additional factors. But, you can't, because homosexuals are a protected group in a way that drug users are not. Whether or not that distinction changes the conclusion, I can't (and am not attempting to) say, only that the argument employed in your last comment doesn't seem to be valid.

@LightnessRacesinOrbit Homosexuals are a protected group because they were made a protected group subsequent to a change in the penal code. The protection is irrelevant. The change in morality is what counts. The situation is not exactly the same but it is analogous.

The relevant factor is that OP had it for personal use, and that does not particularly prejudice him toward commiting other crimes. A better metaphor would be alcohol (as of the Prohibition era). When Prohibition ended, there wasn't an explosion of new crime. People who enjoyed alcohol, deprived of the opportunity to break the law, did not seek other crimes to commit. Just as homosexuals, their orientation now made legal, did not seek perversion into other sexual crime.