The magnetic responses of columnar superstructures of discotic metallomesogens with different alkyl side-chain lengths, cobalt octa(n-alkylthio)porphyrazine (CoSx, where x is the peripheral n-alkyl chain length, x = 10, 12, 14), have been investigated by small-angle neutron scattering (SANS). CoSx (x = 10, 12, 14) were heated to their isotropic phases and cooled down to their columnar mesophases under various external magnetic fields (0.2-1.1 T) and the orientational orderings of the columnar mesophases were measured by SANS. The SANS patterns showed clear anisotropies indicating the alignment of columnar domains with their columnar directors perpendicular to the applied magnetic field. The annularly averaged SANS data, I(), of CoSx (x = 10, 12, 14) under various magnetic field strengths were fitted with Lorentzian functions. The full width at half maximum (FWHM) of the I() of CoS10 and CoS12 rapidly decreases with increasing applied magnetic field and then saturates to about 42° at ~0.5 T and 50° at ~0.6 T, respectively. In the case of CoS14, however, the diffraction anisotropy was very weak even at field strengths as high as 1.13 T and the FWHM was very broad, ca. 120°. The dramatic decrease of magnetic field sensitivity of CoS14 may be attributed to the entropy increase with the side-chain length.