Existing thermal sensors for machining processes are having difficulties to reliably provide high spatial and temporal resolutions for monitoring and control of the dynamic thermal phenomena at the tool-workpiece interface during machining. This paper presents a novel approach to obtain transient tool internal temperature data reliably from a very close distance to the tool-workpiece interface. An array of nine microthin film thermocouples, fabricated using standard microfabrication methods, has been embedded into polycrystalline cubic boron nitride (PCBN) cutting inserts by means of diffusion bonding. Scanning electron microscopy was performed to examine thin film sensor and host material interactions at the bonding interface and to determine optimal bonding parameters. The thin film microthermal sensors were statically and dynamically characterized by heating in a tube furnace and application of pulsed laser radiation. The embedded thin film thermal sensors exhibit good linearity, sensitivity, and extremely fast response time. The instrumented PCBN inserts were applied in machining of aluminum alloy 6061 disks at various cutting speeds and feed rates. Embedded into the PCBN inserts at numerous distances of from tool rake face and from tool flank face, these microsensors enabled the sensing of transient cutting temperature fields with high spatial and temporal resolutions. The temperature data obtained during cutting demonstrate the functionality of the tool embedded microthermal sensors and their value for fast, accurate, and reliable monitoring, which will be of significance for both fundamental study and in-process control of machining operations.