Conventional wisdom identifies human capital and organizational reputation as the critical resources explaining professional partnership (PP) performance. PPs have increasingly adopted organizational practices like strategic planning and formal governance, however, which have long been alien in highly professionalized contexts. In order to test the influence of both these classic resources and the newly adopted practices on PP performance, as well as the mediating mechanisms— that is, client attraction and retention as well as organizational efficiency—through which this influence is channeled, we develop an integrated theoretical framework of PP performance. We test the resulting hypotheses using survey and objective data collected on 196 Dutch law firms. Our findings provide new insights into the drivers of PP performance and the complex interrelationships between PP resources and newly adopted practices.