This study assesses low-cost sensors (LCS) for the mobile monitoring of air quality, which has thus far been scarcely investigated. NO2 and PM2.5 were measured using LCS and higher-grade instruments while driving across various environments in London (943 km) and northern Europe (2923 km), including urban areas, motorways, and tunnels. The data were classified according to the environment where the measurements were carried out, and the performances of LCS and higher-grade instruments were compared. Results indicate that the performances of the sensors were influenced by the rate of change in pollutant concentration in different environments and not by vehicle speed. Excluding tunnel environments, overall, the particulate matter sensors correlated better with their higher-grade instrument than the electrochemical (EC) sensors, with R2 values from 0.90–0.96 in the different environments, compared with 0.39–0.72 for the EC sensors. Tunnels presented a unique opportunity to test the time response of the systems, given the rapid change in concentration upon entering, and all sensors showed limited response times. This is the first time that EC NO2 sensors have been rigorously tested against reference monitors while mobile. Their absolute measurements appear unaffected by movement; however, their time resolution may not be high enough for mobile monitoring in highly variable environments.