We demonstrated the potential of carbon nanoparticles (CNPs) as exogenous contrast agents for both thermoacoustic (TA) tomography (TAT) and photoacoustic (PA) tomography (PAT). In comparison to deionized water, the CNPs provided a four times stronger signal in TAT at 3 GHz. In comparison to blood, The CNPs provided a much stronger signal in PAT over a broad wavelength range of 450-850 nm. Specifically, the maximum signal enhancement in PAT was 9.4 times stronger in the near-infrared window of 635-670 nm. In vivo blood-vessel PA imaging was performed non-invasively on a mouse femoral area. The images, captured after the tail vein injection of CNPs, show a gradual enhancement of the optical absorption in the vessels by up to 230%. The results indicate that CNPs can be potentially used as contrast agents for TAT and PAT to monitor the intravascular or extravascular pathways in clinical applications.