As a low dimensional crystal, graphene attracts great attention as heat dissipation material due to its unique thermal transfer property exceeding the limit of bulk graphite. In this contribution, flexible graphene–carbon fiber composite paper is fabricated by depositing graphene oxide into the carbon fiber precursor followed by carbonization. In this full-carbon architecture, scaffold of one-dimensional carbon fiber is employed as the structural component to reinforce the mechanical strength, while the hierarchically arranged two-dimensional graphene in the framework provides a convenient pathway for in-plane acoustic phonon transmission. The as-obtained hierarchical carbon/carbon composite paper possesses ultra-high in-plane thermal conductivity of 977 W m−1 K−1 and favorable tensile strength of 15.3 MPa. The combined mechanical and thermal performances make the material highly desirable as lateral heat spreader for next-generation commercial portable electronics.