We assessed the joint effects of back pain and mental health conditions on healthcare utilization and costs in a population-based sample of Ontario adults. We included Ontario adult respondents of Canadian Community Health Survey between 2003-2012, followed to 2018 by linking survey data to administrative databases. Joint exposures were self-reported back pain and mental health conditions (fair/poor mental health, mood, anxiety disorder). We built negative binomial, modified Poisson, and linear (log-transformed) models to assess joint effects (effects of two exposures in combination) of comorbid back pain and mental health condition on healthcare utilization, opioid prescription, and costs, adjusting for sociodemographic, health-related and behavioural factors. We evaluated positive additive and multiplicative interaction (synergism) between back pain and mental health conditions with relative-excess risk due-to-interaction (RERI) and ratio of rate-ratios (RR). The cohort (n=147,486) had a mean age of 46 years (SD=17), and 51% were female. We found positive additive and multiplicative interaction between back pain and fair/poor mental health (RERI=0.40;RR=1.12) and mood disorder (RERI=0.41;RR=1.04), but not anxiety for back pain-specific utilization. For opioid prescription, we found positive additive and multiplicative interaction between back pain and fair/poor mental health (RERI=2.71;RR=3.20) and anxiety (RERI=1.60;RR=1.80), and positive additive interaction with mood disorder (RERI=0.74). There was no evidence of synergism for all-cause utilization or costs. Combined effects of back pain and mental health conditions on back pain-specific utilization or opioid prescription were greater than expected, with evidence of synergism. Health services targeting back pain and mental health conditions together may provide greater improvements in outcomes.