Metal oxides with oxygen vacancies can be exploited as a charge storage material for supercapacitor. This work demonstrates synthesis of lanthanum molybdenum oxide (La2Mo2O9, LMO) using different calcination temperatures (300, 400, 500, 600 and 700 °C). As prepared LMO500 illustrates its utilization as an anion intercalated material for supercapacitors. Among all LMOs, LMO500 electrode exhibits high gravimetric (727.18 F g−1 at 0.5 A g−1) and areal (290.87 mF cm−2 at 0.5 A g−1) capacitance. The superior electrochemical performance of LMO500 can be attributed to complete transformation of its mixed phases (i.e. oxides, hydroxides) to its pure oxides (β-La2Mo2O9). Due to existence of pure phase, devoid of secondary phases, LMO500 exhibits good electron kinetics which affords low charge transfer resistance. The asymmetric supercapacitor (LMO500‖LRGONR) employing lacey reduced graphene oxide nanoribbons (LRGONR) as negative electrode delivers high energy density of 52 Wh kg−1 at a power density of 474 W kg−1 which is more than twice of symmetric cell (LMO500‖LMO500). Other than high energy and power density, LMO500‖LRGONR cell shows 98.74% capacitance retention after 1000 GCD cycles at 1.0 A g−1, endorsing excellent cycle stability.