Hydrothermal co-liquefaction of lignite (L) and lignocellulosic biomass represented by cornstalk (CS) and pinewood sawdust (PW) was investigated in this work under various conditions in the presence of formic acid (FA) as an in situ hydrogen donor. The effects of experimental parameters on product distribution and especially the synergistic interaction between lignite and biomass were studied in detail. The results indicated that oil production was significantly enhanced by the addition of FA, while the oil yields obtained from co-liquefaction of L/CS and L/PW reached 29.34 and 28.36 wt % under optimal conditions (350 degrees C, 30 min, lignite/biomass mass ratio = 1:1, with FA), respectively. The synergistic effects between lignite and biomass not only increased the oil yield but also promoted carbon and energy recovery in oil products. The produced oil products from co-liquefaction had similar boiling point fractions as those of the biomass-derived oil, and contained mainly hydrocarbons, phenols, and ketones, indicating that hydrothermal co-liquefaction of lignite and lignocellulosic biomass was beneficial for the production of oil products with high quality and diverse potential applications.