Pardons Canada has been an active voice in the fight for pardons for cannabis offences. If you have been in trouble with the police for pot, marijuanna, cannabis or any other controlled substance then you have a criminal record even if the offence was withdrawn, stayed, dismissed, or discharged.
If you have a criminal record for pot, no matter how small or how old the offence is, then you will get denied entry at the US border. The Americans do random criminal checks so you may have been able to cross the border in the past but it just takes one customs officer to press the criminal check button to see that you have been in trouble and then you will be denied entry to the US.
Once you have been denied entry to the US then you will need a US Entry Waiver to return. This application is expensive and complicated and only lasts for one to five years.
The better option is to remove your criminal record by way of pardon or record suspension. Once this is completed you will be safe for work and travel.
Here is a recent article where Pardons Canada Director was interviewed about the process of removing a criminal record for pot, marijuanna, or cannabis: