On-the-fly evaluation and process control of laser surface microprocessing can be realized with the detection of infrared (IR) thermal radiation. In this study, a new multi-focus mirror was developed and tested for large-area detection of IR-thermal radiation during ultrashort pulsed laser surface micromachining. The IR-thermal detection in the laser micromachining was for the first time combined with a fast hardware signal analysis and laser power control using a field programmable gate array (FPGA) module. The new mirror has a long-line distribution of focused points on one side to collect the emitted thermal radiation along a laser scanning line and focuses the captured IR-thermal radiation to a point on the other side for signal analysis. The length of the detection line of the developed multi-focus mirror was 120 mm and it can be expanded up to tens of centimeters. This IR-thermal radiation signal is processed by the FPGA module with a fast evaluation of heat accumulation at a cycle time loop of 50 ns and on-the-fly control of the laser power during surface microprocessing. When the detected heat accumulation signal grows higher than the required value, then the average laser power is automatically decreased, and vice versa. This lR-thermal tracking system was applied for laser surface micromachining of metals and ceramics with polygon- and galvo-scanners. The presented lR-thermal tracking system with the multi-focus mirror can be used also in other applications, such as beam shaping, line scanning spectroscopy, linear surface irradiation, or non-destructive testing (IR-NDT).