Q: Does a person need a pardon/ record suspension in order to qualify for a Nexus pass?
A: Yes.
Nexus is a joint venture between the USA and Canada to expedite cross border travel for people who are unlikely to pose a risk. Both countries perform background checks to ensure that applicants are not inadmissible under their respective immigration laws. Canada is actually much stricter than the United States regarding who is allowed to enter the country. For example, US Customs officers are normally unconcerned by a Canadian traveler who has one impaired driving conviction whereas Canada Border Services agents are required to refuse entry to a US citizen with the same record. A Canadian citizen with one DWI would probably be approved for Nexus by the US authorities, but not by its Canadian counterpart. This person would have to apply for a pardon/ record suspension in order to meet the Canadian criteria for Nexus.
Most convictions will get you barred on both sides. Drug possession is a good example. A Canadian citizen with this type of conviction would have to apply for a pardon to get approval for Nexus on the Canadian side but they would still fail the US criteria. The Canadian government recognizes pardons/ record suspensions but unfortunately the US administration does not. This person would have to get a waiver of inadmissibility before applying for Nexus. Nexus could be refused even with a waiver in place.
Long story short: a pardon or record suspension is always required by the Canadians to grant a Nexus pass but it may not be enough to satisfy the American requirements. If you have any convictions from your past (regardless of whether a pardon has been granted) a Nexus card may be a difficult document to obtain.