Objective: Little is known about changes in alcohol and cannabis co-use over time in adolescents and young adults. We sought to describe the natural course of alcohol and cannabis co-use from age 12 to 17 and 20 to 31 and to describe frequent (i.e., ≥ once/month) binge drinking (i.e., ≥ 5 drinks/occasion) and cannabis use from age 20 to 31. Method: Data were drawn from two longitudinal studies conducted in Montréal, Canada: AdoQuest (n = 1,852) and the Nicotine Dependence in Teens (NDIT) study (n = 1,293). Complete data on alcohol and cannabis were available for 620 12–17-year-olds (58% female) followed from 2005 to 2011 in AdoQuest, and 673 20–31-year-olds (56% female) followed from 2007 to 2020 in NDIT. We reported proportions of co-use, exclusive drinking (“drinking”), exclusive cannabis use (“cannabis use”), frequent co-use, frequent binge drinking, and frequent cannabis use by age and sex. Results: Co-use rose from 2% at age 12–13 to 23% at age 17, was stable at 44% at age 20 and 24, and then decreased to 37% at age 31. Drinking rose from 20% to 51%, and cannabis use was consistently rare (