In the search for new therapeutic strategies for the treatment of skin cancer, cannabinoids have become the focus of scientific interest. The present study investigated the effects of the phytocannabinoids Δ⁹-tetrahydrocannabinol (THC) and cannabidiol (CBD) on the viability, apoptosis, and mitochondrial function of human melanoma (A375) and cutaneous squamous cell carcinoma (SCC) cells (A431). Both cannabinoids caused a time- and concentration-dependent loss of viability and an upregulation of caspase-3/7 activity, associated with the induction of initiator caspases-8 and -9, PARP cleavage, and an increase in the autophagy marker LC3A/B-II. Inspired by the latest work on the dual role of heme oxygenase-1 (HO-1) in cell fate, the expression of this enzyme was examined and found to be upregulated at the mRNA and protein level by THC and CBD. Inhibition of HO-1 activity by tin protoporphyrin IX (SnPPIX) reduced the loss of viability caused by both cannabinoids, suggesting a cytotoxic rather than cytoprotective mediator role for this enzyme here. At the mitochondrial level, THC and CBD caused a reduction in membrane potential, a release of cytochrome c into the cytosol, and electron microscopically detectable mitochondrial damages. A more detailed functional analysis revealed an inhibition of mitochondrial oxygen consumption rate, accompanied by a decrease in various subunits of mitochondrial oxidative phosphorylation complexes. In conclusion, our data demonstrate a strong cytotoxic effect of THC and CBD on melanoma and cutaneous SCC cells involving mitochondrial apoptosis and mitochondrial dysfunction.