Adolescence is a critical phase in an individual’s development. Adolescents are especially susceptible to the impacts of external environment stimuli (e.g., afforded social opportunities, area walkability, ambient pollution), yet relatively little research has sought to quantify how urban design exposures might affect their mental health. To explore the links between specific urban design features and adolescent mental health, we quantified the pedestrian- and transit-oriented design (PTOD) quality of six different urban areas using a validated tool, and presented videos of these areas to 1,500 Canadian adolescents via an online survey. Outcomes of interests were participants’ emotional responses to each video (i.e., positive affect, negative affect, calmness, anxiousness, perceived restorativeness, and mental demand). Linear mixed models were run and then adjusted for a variety of sociodemographic and environmental controls, with the subsequent findings showing several significant associations between increases in the scores of the different PTOD concepts and the reported adolescent emotional responses. While the findings are correlational in nature, the design concept transparency was associated with the greatest increases in positive affect (β = 1.442, 95% CI: 1.322, 1.562), calmness (β = 0.540, 95% CI: 0.410, 0.669), and perceived restorativeness (β = 1.976, 95% CI: 1.792, 2.159). Scale was associated with significant increases in positive affect (β = 0.656, 95% CI: 0.608, 0.704) and decreases in negative affect (β = −0.202, 95% CI: −0.239, −0.164), as was complexity with both positive affect (β = 0.429, 95% CI: 0.396, 0.461), and negative affect (β = −0.148, 95% CI: −0.174, −0.123). To support adolescent mental well-being through design, urban designers should consider scale, complexity, and transparency concepts in their design guidelines and building codes. Future research should look to examine these relationships via experimental designs and externally valid methods (e.g., ecological momentary assessments).