Euphorbia supina has been traditionally used for the treatment of furuncle and bloody diarrhea relevant to the inflammatory process. It has been proven to have a variety of pharmacological efficacies including antiarthritic, detoxification, hemostatic, and diuretic activities. RAW 264.7 macrophages and bone marrow-derived mast cells (BMMCs) were used to determine the anti-inflammatory and anti-allergic effects of E. supina (ES). NO production was assayed by measuring the nitrite content of the supernatants of cultured RAW 264.7 cells. β-hexosaminidase, a marker of mast cell degranulation, was quantitated by spectrophotometric analysis. ELISA was used for the analysis of interleukin-6 expression, and Western blotting was used to analyze 5-LOX, iNOS, and MAPK activation. The relevant gene expression upon ES treatment was measured by RT-PCR. ES inhibited inducible nitric oxide synthase (iNOS) in RAW 264.7 cells, and IL-6 and LTC4 production in PMA- and A23187-induced BMMCs along with the downregulation of 5-LOX gene expression. Furthermore, in the present study, a decrease in p-ERK, p-JNK, and p-P38 expression, as well as the suppression of degranulation, were observed by treatment with ES. Further in vivo study revealed that ES treatment also remarkably inhibited xylene-induced mouse ear edema and MPO levels in mice ears. This study demonstrates that ES has a potential regulatory effect on the expression of inflammatory mediators through the inhibition of both the phosphorylation of MAPK signaling and the activation of degranulation.