Premature closure of mentoring relationships decreases positive effects of mentoring or can even lead to negative effects for mentees. Past studies retrospectively investigated mechanisms of premature match closure. However, a deeper understanding of the dynamics that lead to premature match closure is still missing. In our study, we longitudinally examined the preprogram characteristics, program adherence, as well as program communication and networking behavior of girls (N = 901, M = 13.80 years) who took part in a 1-year online mentoring program in science, technology, engineering and mathematics (STEM), comparing girls who dropped out of the program prematurely (N = 598) with girls who were considered as non-dropouts (N = 303). We used survival analysis methods to simultaneously analyze time-independent characteristics and time-dependent dynamics of mentees' communication and networking behavior. Besides mentees' interest in STEM and compliance with program specifications, a frequent and steady communication with their mentors decreased the risk for premature match closure, especially, if it focused on STEM. Mentors' mentoring experience, mentees' program-wide networking and their networking with other mentees reduced the risk for premature match closure. Regarding the STEM focus of networking, we found competing influences, which need to be further explored in future research.