Mumps virus (MuV) encodes a phosphoprotein (P) that is important for viral RNA synthesis. P forms the viral RNA-dependent RNA polymerase with the large protein (L). P also interacts with the viral nucleocapsid protein (NP) and self-associates to form a homotetramer. The P protein consists of three domains, N-terminal domain (PN), oligomerization domain (PO) and C-terminal domain (PC). While PN is known to relax the NP-bound RNA genome, the roles of PO and PC are not clear. In this study, we investigated the roles of PO and PC in viral RNA synthesis using mutational analysis and a minigenome system. We have found that PN and PC functions can be trans-complemented. However, this complementation requires PO, indicating that PO is essential for P function. Using this trans-complement system, we have found that P forms parallel dimers (PN to PN and PC to PC). Furthermore, we have found that residues R231, K238, K253 and K260 in the Po were critical for P's functions. We have identified PC as the domain that interacts with L. These results provide structure-function insights into the role of MuV P. SIGNIFICANCE: MuV, a paramyxovirus, is an important human pathogen. The P protein of MuV is critical for viral RNA synthesis. In this work, we established a novel minigenome system that allows domains of P to complement in trans. Using this system, we confirmed that MuV P forms parallel dimers. Understanding viral RNA synthesis will allow design of better vaccines and development of antivirals.