Common ragweed (Ambrosia artemisiifolia L.) is frequently observed in Québec carrot fields. Carrot growers essentially rely on linuron, a photosystem II inhibitor, to control this broadleaf weed. A linuron-resistant biotype had been identified but its prevalence was unknown and the genetic basis of resistance was not established. Consequently, a survey was conducted and plants suspected to be resistant were collected in 2012 and 2013. Progeny from these plants were sprayed with a diagnostic rate of linuron and tested for the presence of target site mutations in the psbA gene. Common ragweed was the most reported species (95% of accounts) and 94% of populations were diagnosed as resistant. A new target site mutation was found in 37.5% of resistant populations tested. No mutations in the psbA gene known to confer resistance to linuron were found in the other resistant populations. Except for two populations, target site resistant plants were located in the muck soil production area, while those diagnosed as non-target site resistant were found in sandy fields located in a different area. To our knowledge, this is the first report of a Val₂₁₉Ile mutation in the psbA gene of common ragweed and of evolved non-target site resistance to linuron.