Background: When a patient is incapable of making medical decisions for him-or herself, a substitute decision-maker makes choices according to the patient's previously expressed wishes, values and beliefs; however, little is known about public readiness to act as a substitute decision-maker in Canada. Our primary objective was to measure public self-reported preparedness to act as a substitute decision-maker, and explore the attitudes, barriers and enablers associated with preparedness. Methods: From November 2017 to June 2018, we conducted a mixed-methods street intercept survey at 12 pedestrian areas in Ottawa, Ontario. We used descriptive statistics and logistic regression analysis to assess predictors of perceived preparedness to be a substitute decision-maker and determine support for high school education. We analyzed qualitative interview questions using inductive thematic analysis. Results: Of the 626 eligible respondents, 196 refused to participate, leaving 430 participants (response rate 68.7%). A total of 404 surveys (94.0%) were fully complete with no missing data. The respondents were mostly female (243 [56.5%]) and residents of Ontario (364 [84.6%]). The average age was 33.9 years. Although 314 respondents (73.0%) felt prepared to be a substitute decisionmaker, 194 (45.1%) reported never having had meaningful conversations with loved ones to understand their wishes in the event of critical illness. A total of 293 participants (68.1%) identified important barriers to feeling prepared. Most respondents (309 [71.9%]) agreed that high school students should learn about being a substitute decision-maker, citing age appropriateness, potential societal benefit and improved decision-making, while cautioning the need to respect different maturity levels, cultures and experiences. Interpretation: The lack of conversation between loved ones reveals a gap between perceived and actual preparedness to be a substitute decision-maker for a loved one with a critical illness. The overall acceptability of high school education warrants further exploration.