Since the terrorist attacks of 11 September 2001 and the establishment of the US Department of Homeland Security (DHS), foreigners travelling to the US have had to face decidedly more stringent scrutiny when attempting to enter through the Canadian border than used to be the case. No longer can a person with a Canadian license plate or identification hope to be kindly waved across the border; passport control and a background check are now standard procedure. And that spells danger for people with a criminal conviction to their name: it is illegal for anyone with a criminal record to travel to the US unless they have a valid US entry waiver. At the border, a guard will check a person's identity against records provided by the Canadian Police Information Centre (CPIC); if the person is found to have a Canadian criminal record, they will be sent back and banned from entering the US for five years. They will also be liable to having their vehicle seized and being charged with illegal entry.
Does having one's conviction pardoned make any difference? Not really. The DHS does not recognize Canadian pardons, and while the CPIC will not provide the US with information on pardoned convictions, the DHS may still obtain the information before the pardon is granted or in some other manner. We live in an electronic age in which a great mass of information is readily available and if a past conviction does not come to light during one border crossing, it may at the next. Allen Abney, who deserted the US Marines and moved to Canada in 1968 to avoid being sent to Vietnam, is a textbook example of how the past can unexpectedly come to light. In 2006, after years of travelling back and forth between Canada and the US with no problems, he was identified as a deserter during a background check at the border and had to spend a brief period of time in the Camp Pendleton military prison before being released with a general discharge. So even if someone has crossed the border multiple times without being stopped, there is no guarantee that they won't be in the future.
For some, it might be tempting to circumvent the authorities by entering the US through a forest or other natural boundary where there is no passport control. This is both illegal and exceedingly dangerous. Although the Canada-US border has a reputation for being significantly less patrolled than the US-Mexico border, in the post-9-11 era, it is far easier to be caught crossing than it used to be. Today, a border guard need not be physically present; there are motion detectors hidden at different points along the border that can sense suspicious movement and alert authorities. Those who are caught entering the US illegally can expect to be arrested and charged.
The only legitimate avenue for those who have a criminal conviction and wish to travel to the US is to apply to the DHS for a US entry waiver. If a person's only conviction is for a single summary offence (less serious crime) or for reckless driving or driving under the influence, they do not need to obtain an entry waiver. Otherwise, they need to submit their application well ahead of when they plan to travel, as it can take the DHS up to 10 months to process it. One must also bear in mind that a waiver is not granted for life; it is valid for one to five years and then it must be renewed.