Directed energy deposition (DED), also known as laser cladding, is one type of additive manufacturing where a high-power laser combined with a coaxial powder delivery system is used to produce metal components layer-by-layer. Despite many advancements in DED, little work has been accomplished in online estimation of mechanical properties, such as Young's modulus and Poisson's ratio, during the DED process. In this study, an online mechanical properties estimation technique was developed for the DED process using femtosecond laser ultrasonics and laser polishing. The unique contributions of this study are the following: (1) the development of a fully noncontact and nondestructive technique for the estimation of Young's modulus and Poisson's ratio using a femtosecond laser, (2) layer-by-layer estimation of mechanical properties accomplished by achieving micrometer-level spatial resolution, (3) laser ultrasonic measurement from a deposited metal layer using laser polishing instead of mechanical polishing, and (4) potential for online operation during the DED process. The performance of the proposed technique was examined using specimens fabricated under various DED operation conditions. The Young's modulus and Poisson's ratio values estimated by the proposed technique were consistent with those obtained from independent tensile tests performed after the completion of the DED process.