I am often asked by clients whether a particular charge or conviction will prohibit them from entering the USA. Regrettably the law on this issue is not clear. The United States prohibits anyone from Canada from entering their country if they have been convicted of any crime of moral turpitude. Unfortunately the law does not clearly define what moral turpitude is. However, there is a wide variety of cases in the US which have made that determination. Unfortunately criminal law in the US is a state matter not a federal matter so often the determination of moral turpitude for an offense is contradictory depending on which state made that decision and Canadian criminal offenses are not necessarily the same as US offenses. In the end it is up to the guards at the border to make that determination and they have wide discretion.
That being said there are some offenses that usually raise a red flag. Offenses involving dishonesty such as theft, fraud, perjury, embezzlement, forgery, break and enter etc. are generally considered crimes of moral turpitude and admittance into the US would be denied. Likewise serious offenses such as robbery, attempted murder, murder and the like are also entrance prohibited crimes. Finally the US prohibits anyone from entering convicted of any drug or sexual offenses.
Impaired driving and weapon offenses are generally not cause for denial of admission into the US , ironically however if you are a US citizen trying to enter Canada with a record for these types of offenses we would deny you admission to our country. Similarly Canada does admit US citizens to enter our country with a minor drug record. Crimes of violence such as assault and assault bodily harm normally do not trigger a ban however assault with a weapon, assault police officers and aggravated assault do. Most driving offences including dangerous or prohibited driving are also not cause for border stoppage unless that driving caused bodily harm or death in which case you would be denied admission.
As you can see from the above article the rules for admission vary greatly. I have listed many of the more common offenses and generally how that would affect your admission into the US. However, these are guidelines not rules and the guard at the border you deal with has wide discretion to admit or deny admission even if your record is an offense that usually doesn't cause a problem. If you are denied admission into the US your name is then red flagged and the only way to gain entrance is to apply for a waiver through the US government. Generally waivers are granted if the offenses are less serious and dated however there is a fee for this waiver which can amount to several hundred dollars and the waiver is usually only good for six months and then needs to be renewed and the fee paid again etc. etc. It should be noted when crossing the border that the US has access to our records and the act of denying a criminal record that does exist even if it's for an offense that is not of moral turpitude is still considered grounds for denial into their country.
Finally the US does not acknowledge our system of pardons and if you've entered the US in the past and they have ran your record on their system it will always be there. They will not recognize or acknowledge you've received a pardon by the Canadian government.
Sadly the rules regarding admission into the US are clear as mud and if you do have a record and are attempting to cross the border - don't lie about it and good luck!