Recent studies suggest isocitrate dehydrogenase 1 (IDH1) mutations are early events in gliomagenesis, given their frequent occurrence in low-grade gliomas, diffuse expression within neoplastic cells, and lack of evidence for preceding TP53 mutations or 1p/19q co-deletion. We present an infiltrating glioma with mixed oligoastroglial morphology and biphasic molecular phenotype. Areas resembling oligodendroglioma by histology expressed mutant IDH1-R132H, and strong ATRX, Olig2, and PDGFR-α by immunohistochemistry. In contrast, astrocytic areas completely lacked the IDH1-R132H mutation, showed loss of nuclear ATRX expression, and only weakly expressed Olig2 and PDGFR-α. Co-deletion of 1p/19q was evident throughout, while p53 expression was largely negative. This case suggests that 1p/19q co-deletion may rarely precede IDH1 mutations or that IDH1 mutations may be secondarily lost, as demonstrated by IDH1-R132H positive and negative cells in a glioma with diffuse 1p/19q co-deletion. The uniquely biphasic molecular phenotype of this tumor supports the rare existence of true mixed oligoastrocytomas that may have significant prognostic and therapeutic implications. The case highlights the variable sequence of key molecular aberrations in gliomagenesis and the difficulty of targeting treatment to genetic profiles in inherently heterogeneous neoplasms.