The 12 C(α, γ) 16 O reaction is one of the most crucial reactions in nuclear astrophysics. The E2 external capture to the O 16 ground state (GS) has not been emphasized in previous analyses but may make a significant contribution to the 12 C(α, γ) 16 O cross section depending on the value of the GS asymptotic normalization coefficient (ANC). In the present work, we determine this ANC to be 337±45 fm −1/2 through the 12 C( 11 B, 7 Li) 16 O reaction using a high-precision magnetic spectrograph. This sheds light on the existing large discrepancy of more than two orders of magnitude between the previously reported ANC values. Based on the new ANC, we experimentally constrain the GS external capture and show that through interference with the high energy tail of the 2 + subthreshold state, a substantial enhancement in the GS SE2(300) factor can be obtained (70±7 keV b) compared to that of a recent review (45 keV b), resulting in an increase of the total S-factor from 140 keV b to 162 keV b, which is now in good agreement with the value obtained by reproducing supernova nucleosynthesis calculations with the solar-system abundances. This work emphasizes that the external capture contribution for the ground state transition cannot be neglected in future analyses of the 12 C(α, γ) 16 O reaction.