In this work, we report a novel, cost effective and environmental-friendly green synthesis method of reduced graphene oxide (rGO) – tin oxide (SnO2) composite using lemon extract by hydrothermal process for selective and ultra-sensitive electrochemical detection of ascorbic acid. The electrochemical behavior of ascorbic acid at the surface of rGO-SnO2 composite modified glassy carbon electrode (GCE) was investigated by cyclic voltammetry (CV) technique whilst ascorbic acid sensing was performed by using differential pulse voltammetry (DPV) technique. An excellent sensitivity of 1200 μA mM−1 cm−2 (R2 = 0.990) towards ascorbic acid in the wide dynamic range of 400 μM −1600 μM without any surface fouling effect was observed which is, to the best of our knowledge, ~1.6 folds higher than previously reported ascorbic acid sensors. The as-fabricated sensor exhibited lower limit of detection (38.7 μM), superior reproducibility and selectivity towards interfering species such as glucose, urea, uric acid and Na+ ions. This enhanced sensing ability was studied and discussed in terms of forward biased nano-Schottky barriers at the rGO-SnO2 interface. The sensor was successfully evaluated for the detection of ascorbic acid in the vitamin C tablets and to predict unknown concentrations of ascorbic acid in PBS solution. This ultrasensitive, binder free rGO–SnO2 composite based sensor can be used for a simple, low cost and voltammetric detection of ascorbic acid in clinical diagnostics, pharmaceutical and food industries.