A protean career orientation is assumed to be beneficial for career development but researchers have only recently started to empirically evaluate the concept. Conducting two studies based on three independent samples of university students and working professionals in Germany, we address issues of concurrent validity, predictive incremental validity and mechanisms linking the protean orientation to career outcomes. The first study showed that in a sample of 104 German employees different measures of the protean career orientation all correlated highly, but not identically, to a range of work and career attitudes. Using bootstrapping analysis, a second study with a six-month prospective examination among 419 German university students and a cross-sectional analysis among 526 German employees showed that a protean career orientation predicts proactive career behaviors and career satisfaction beyond a proactive disposition and core self-evaluations, respectively. Moreover, the protean career orientation was a significant mediator of these two personality constructs on both career outcomes. Cumulatively, the studies enrich our understanding of how and when a protean career orientation is related to important career outcomes.