We report on the fabrication and transport characterization of atomically-precise single molecule devices consisting of a magnetic porphyrin covalently wired by graphene nanoribbon electrodes. The tip of a scanning tunneling microscope was utilized to contact the end of a GNR-porphyrin-GNR hybrid system and create a molecular bridge between tip and sample for transport measurements. Electrons tunneling through the suspended molecular heterostructure excited the spin multiplet of the magnetic porphyrin. The detachment of certain spin-centers from the surface shifted their spin-carrying orbitals away from an on-surface mixed-valence configuration, recovering its original spin state. The existence of spin-polarized resonances in the free-standing systems and their electrical addressability is the fundamental step for utilization of carbon-based materials as functional molecular spintronics systems.Single molecule spintronics envisions utilizing electronic spins of single molecules for performing active logical operations. The realization of such fundamental quantum devices relies on the accessibility of electronic currents to the active molecular element, and on the existence of efficient electron-spin interaction enabling writing and reading information. Single molecule (SM) electrical addressability has been achieved in break junctions' experiments [1-3], which realized that the nature of the SM's contacts to source-drain electrodes may affect the ultimate SM functionality [4]. To ensure stable and reproducible behaviour of the SM device, robust and atomically precise molecule-electrode contacts with optimal electronic transmission are required [5,6]. Aromatic carbon systems such as nanotubes, and graphene flakes are considered ideal electrode materials [7,8] because of their structural stability and flexibility, and the large electronic mobility they exhibit. Graphene electrodes also offer the perspective of covalently bonding to a single molecule at specifically designed sites [9], thus creating robust systems for electrical measurements.While methods to fabricate graphene-SM systems are being developed [10,11], the spin addressability by electronic currents through them remains to be demonstrated. Attractive predictions on the role of graphene-SM hybrids as spin valves, diodes, or rectifiers [12,13] are proposed to strongly depend on the coupling of electronic currents with spin-polarized molecular states and, hence, on the precise contact geometry. Furthermore, spins are sensitive to electronic screening [14] and to electrostatic and magnetic fields [15] in its local environment. Therefore, precise strategies for spin detection and manipulation in atomically controlled hybrid structures are needed [16] to determine the magnetic functionality of the hybrids and the possible operation mechanisms.In this work, we electrically address the spin of an iron porphyrin molecule covalently wired to graphene nanoribbon (GNR) electrodes. To perform two-terminal transport measurements with atomic-scale control on the GNR-S...