This study examines the effect of using Desmos on students’ performance in learning functions. An embedded mixed methods design was applied and involved 98 students from an upper secondary school in Sweden. Students’ assessments (pre- and post-test) and opinion polls were the two main data collection instruments. The results show that both groups (experimental and control) experienced a significant improvement in their post-test scores. However, the experimental group had a statistically significant improvement in comparison with that of the control group across the five constructs used in this study. The qualitative data revealed that the majority of the students ascribed a positive effect of the use of Desmos on their general understanding of function concepts, their ability to analyze functions and check their answers through visualization, which are difficult when working using paper and pencil.