Americans with a single criminal conviction that is not serious (maximum imprisonment of equivalent crime in Canada is less than ten years) may be allowed back into Canada if it has been more than a decade since full completion of all sentencing including any probation. For example, a person with one minor theft conviction for shoplifting may be considered deemed rehabilitated after ten years. To avoid the risk of a border refusal, however, court documents or other paperwork should be brought with you to Canada as old convictions can still raise questions at the border. Any criminal offense that involved violence, a weapon, or property damage does not qualify for Deemed Rehabilitation, and can render a person criminally inadmissible to Canada for life. A DUI is now considered a serious crime in Canada, and any such offense can now result in an American being refused entry at the Canadian border even if it happened more than ten years ago.
quoted from https://www.temporaryresidentpermitcanada.com/crimes-that-make-you-inadmissible.php