As a US citizen you may qualify to participate in Nexus.
Please note you may not qualify if you intentionally provide false or incomplete information on your application, have been convicted of a criminal offence or you have a recorded violation of customs, immigration or agriculture law.
Both Canada and the United States must approve your application for you to become a member of NEXUS. If you do not meet all of the requirements, your application will be denied.
Regarding a work permit for Canada you may be exempt; it is recommended you contact your local Canada visa office: