Science, technology, engineering, and mathematics (STEM) outreach programs aim to increase participation in STEM fields. However, the impact of these programs is rarely measured due to inherent difficulties in conducting long-term evaluations. This article presents a decadal evaluation of the Science and Engineering Challenge (SEC), an Australian STEM outreach program. From 2006 to 2015, 5,210 high school and 2,445 first-year university students were surveyed to assess whether the SEC influenced their decision to pursue STEM studies. Of the high school physics students, 51.9% reported that the SEC influenced their decision to study physics. A smaller yet significant impact was reported by chemistry (35.2%) and mathematics (32.0%) students. Further, 30.9% of university students indicated that the SEC influenced their decision to pursue a STEM degree. These findings demonstrate that long-term evaluation of outreach program effects is achievable and that outreach programs can indeed have a demonstrable impact on student career choices.