She is the founder of the Share the Road Cycling Coalition, an urban transportation policy and advocacy group she created following the death of her husband OPP Sergeant Greg Stobbart, in a collision in June 2006.

[2] She was employed on Parliament Hill as press secretary successively for Herb Gray, John Turner and Jean Chrétien.

She pressed the government to amend the Highway Traffic Act to toughen the law regarding drivers who drive with suspended licences.

McMahon led the passage of two important amendments to the Highway Traffic Act, stemming from a Private Member's Bill she tabled in 2016.

If a motorist is convicted of Careless Driving involving a pedestrian or cyclist, the penalties are enhanced due to their vulnerability.