Geometry is essential for mathematics learning given that it is strongly related to our surroundings; however, few studies concentrated on using geometry in our daily life, especially using mobile devices with their sensors. Thus, this study proposed one app, Ubiquitous Geometry (UG), and explored its effects on learning angles and polygons in authentic contexts. The experiment was conducted for grade four learners of an elementary school. The control group used protractors and pencil/paper in measuring angles and polygons, whereas the experimental group did measurements with UG. The results showed that in terms of learning achievement, the experimental group outperformed the control group. Further investigation of the relationship between learning behaviors and learning achievement in the experimental group found that both learning effectiveness and quantity of learning, including measuring angles of elevation and depression (MED), note drawing, and comment drawing, have significantly positive correlations with learning achievement. These three behaviors also become significant predictors of learning achievement after multiple regression analysis. Moreover, MED was found to be the most critical factor to affect learning achievement. Additionally, in perception evaluation, participants felt satisfied with UG and authentic measurement activities by which their learning motivation and interests in authentic contexts were indeed stimulated. Hence, we suggested that UG was worth promoted and further investigated its effects on authentic geometry learning.