Hello I am a solicitor with 20 years experience. I will try to help you with this.
It is hard sometimes to know about the precise effect an old conviction will have on an application for a visitor visa. Offences which frequently prevent entry to Canada include: drink driving or impaired driving convictions (even if they are recorded as a traffic offence) where the blood alcohol reading is 0.081% or above – or a breath reading of 0401mgms/L or above, will make you inadmissible to Canada. Other common convictions that may make a person inadmissible to Canada are: reckless or dangerous driving, common assault, street racing, hinder or resist a police officer in the execution of duty, possession, supply, trafficking of drugs (including cannabis) and shoplifting (theft), fraud or criminal damage, to name a few.
After a prescribed period of five (5) years has passed since the completion of any imposed sentence (and there have been no other convictions that affect your admissibility) an application for Criminal Rehabilitation may be lodged. If approved, Rehabilitation will overcome the inadmissibility permanently.
If you only have one conviction of a less serious nature, after a prescribed period of ten (10) years has passed since the completion of any imposed sentence (and there have been no other convictions that affect your admissibility) you will be deemed to be rehabilitated pursuant to R18(2) of Canada's Immigration Regulations. You do not have to contact this office. If you have more than one offence, deemed rehabilitation may not apply to your case and you will require a formal assessment.
As far as entry to the USA is concerned this can be problematic if one has a conviction. What was the conviction for?
I don't think the clean record in Britain is particularly relevant bearing mind the US conviction.