Ultrafast photothermal treatment by flash light can effectively reduce graphene oxide liquid crystalline fibers. Resultant flash induced graphene fibers with different reduction conditions enable delicate controllability of the mechanical properties and electrical conductivity along with spatial control of reduction level. Humidity sensors are fabricated from flash reduced graphene fibers and reveal superior sensing performance over conventional thermally reduced counterparts. Abstract Flash photothermal treatment via Xenon lamp with a broad wavelength spectrum can effectively remove oxygen functionalities and restore sp2 domains at graphitic carbon materials. The chemical composition and relevant structure formation of flash reduced graphene oxide liquid crystal (GOLC) fibers are investigated in accordance with flash irradiation conditions. Owing to the spatial controllability of reduction level via anisotropic flash irradiation, the mechanical properties and electrical conductivity of graphene fibers can be delicately counterbalanced to attain desired properties. High sensitivity humidity sensors can be fabricated from the flash reduced fibers demonstrating notably higher sensitivity over the thermally reduced counterparts. This ultrafast flash reduction holds great promise for multidimensional macroscopic GO based structures, enabling a wide range of potential applications, including textile electronics and wearable sensors.