In the field of second language (L2) writing, we have recently seen "a collective future trend towards methodological diversity and expansion of repertoires of research methods and approaches" (Manch?n & Matsuda, 2016, p. 9). In responding to the call for methodological diversity, we set out to explore mixed methods research methodology in L2 writing research. Mixed methods research (MMR) has been acknowledged as a 'third' or 'alternative' methodological approach along with quantitative and qualitative research approaches over the past two decades in social sciences and other fields. However, relatively little has been discussed with regard to MMR in the field of L2 writing. More importantly, well-designed, rigorous MMR can have a great potential to provide a holistic understanding of complex issues in L2 writing research. Given these, in this article, we report the findings from our systematic synthesis of 27 empirical MMR in L2 writing with respect to 'research contexts/participants,' 'research topics,' 'purpose of MMR,' 'data sources,' and 'structure of MMR.' Our research synthesis provides L2 writing researchers with some valuable insights into the trend of and future directions for MMR in L2 writing.