Inspired by the numerous benefits of mentorship in academia, we study "informal mentorship" in scientific collaborations, whereby a junior scientist is supported by multiple senior collaborators, without them necessarily having any formal supervisory roles. To this end, we analyze 2.5 million unique pairs of mentor-protégés spanning 9 disciplines and over a century of research, and we show that mentorship quality has a causal effect on the scientific impact of the papers written by the protégé post mentorship. This effect increases with the number of mentors, and persists over time, across disciplines and university ranks. The effect also increases with the academic age of the mentors until they reach 30 years of experience, after which it starts to decrease. Furthermore, we study how the gender of both the mentors and their protégé affect not only the impact of the protégé post mentorship, but also the citation gain of the mentors during the mentorship experience with their protégé. We find that increasing the proportion of female mentors decreases the impact of the protégé, while also compromising the gain of female mentors. While current policies that have been encouraging junior females to be mentored by senior females have been instrumental in retaining women in science, our findings suggest that the impact of women who remain in academia may increase by encouraging opposite-gender mentorships instead.