The US does not recognize a Canadian pardon. However, if you get a pardon, your record will be removed from the Canadian police system and the US customs agent won't be able to find it. However, if they had previously found it on some previous entry before the pardon, then they have copied it into their system and its removal from the Canadian system won't matter, as they will still have their copy of the data. In that case they know about it, and the fact that you have a pardon from Canada is irrelevant to them--you will need a pardon from US Customs too.
What's even worse is that the US considers a Canadian "absolute discharge" or "conditional discharge" to be a registered criminal conviction. Technically it is: With a discharge a judge finds you guilty, but decides it isn't worth punishing you. All the US cares about is the part where the judge found you guilty--that you weren't actually punished is irrelevant, they will still exclude you.
So, a lot of people in Canada who commit relatiely minor crimes and are granted a discharge by the judge feel that the matter is behind them. Then they try and enter the US, and they are told that they are inadmissible because they have a criminal conviction. Confusion ensues.
The short story is if you have had ANY kind of run in with the law you should seek the help of a US immigration attorney before trying to enter the US. Key word there is BEFORE. Most of these problems are harder to fix if you learn about them at the border from US customs.