Knowing the temporal direction of causal relations is critical for producing desired outcomes and explaining events. Existing evidence suggests that children start to grasp that causes must precede their effects (the temporal priority principle) by age 3; however, whether younger children also understand this has, to our knowledge, not previously been tested. Given the importance of temporal priority in making sense of the world, we explored when knowledge of this principle develops. In the present study, conducted in a lab or museum in a Canadian city, 1- and 2-year-old children observed an adult perform action A on a puzzle box (e.g., spinning a dial), following which an effect E occurred (a sticker was dispensed), following which the adult performed action B (e.g., pushing a button; A-E-B sequence). In line with the temporal priority principle, toddlers were significantly more likely to manipulate A than B (Experiment 1, N = 41, 22 female), even when A was spatially disconnected from the sticker dispenser and further from it than action B (Experiment 2, N = 42, 25 female). In Experiment 3 (N = 50, 25 female), toddlers observed an A-B-E sequence such that both actions A and B were performed prior to effect E. Here, they primarily intervened on B, which ruled out that success in Experiments 1-2 was based on a primacy effect. A lack of any age effects across experiments suggests that within the second year of life, children already grasp that causes must precede their effects, providing key insights into causal reasoning in early childhood.