It is essentially illegal to attempt to enter the U.S. with a criminal record, unless you have appropriate immigration status or you have a U.S. Entry Waiver.
Haven’t Been Caught Yet?
You may have often passed through the U.S. Customs & Border Protection (CBP) after answering a few standard questions about your citizenship and the purpose of your trip. You have been lucky. It is becoming more common for the CBP to ask for identification for the purpose of conducting an RCMP computer criminal record search.
Once your record has been discovered, the CBP officer will download your criminal record into the U.S. computer system, and as a result, you will have a CBP and an FBI file. Once you are denied access to the U.S., you will be required to apply for a U.S. entry permit called a U.S. Entry Waiver.