No matter how old and how small your drug offence is, the Americans will deny you entry to the US.
You may have had luck traveling in the past as the Americans only do random criminal checks, so they may not have done a check on you. However, criminal checks at the US border are becoming more consistent especially as the Canadian government moves to legalize marijuana.
If you have already been denied entry to the US, then you will need a US Entry Waiver or I-192 to return to the US. This is an expensive and complicated application and can only last between 1-5 years and then you will need to renew the application.
The good news is that if you have not been denied entry to the US, then once your pardon is granted and your criminal record is removed, then you should be safe to travel without the Americans seeing your offence.
The following article is very cohesive in explaining the ramifications of traveling to the US with a marijuana offence.
It includes the answers to the following questions:
Will I be barred for using/possessing marijuana in Canada after it becomes legal?
Will I be barred for a marijuana possession conviction or an admission of prior use that occurred before the Cannabis Act came into force?
I’m employed in the marijuana industry in Canada. Can I be denied admission to the US because of my occupation?
Will I be barred if I smoked marijuana during my trip to Colorado or California?
I am a Canadian citizen who has been hired by a US based company involved in the marijuana industry, to work in a US state that has legalized pot. Will I have problems when I apply for a work permit?