Understanding complex neuronal networks requires monitoring long-term neuronal activity in various regions of the brain. Significant progress has been made in multi-site implantations of well-designed probes, such as multi-site implantation of Si-based and polymer-based probes. However, these multi-probe strategies have been limited by the sizes and weights of interfaces to the multiple probes and the inability to track the activity of the same neurons and changes in neuronal activity over longer time periods. Here, we report a long single flexible probe that can be implanted by stitching into multiple regions of the mouse brain and subsequently transmit chronically-stable neuronal signals from the multiple sites via a single low-mass interface. We implanted the probe at four different sites using a glass capillary needle or two sites using an ultrathin metal needle. In-vitro tests in brain-mimicking hydrogel showed that multi-site probe implantations achieved a high connection yield of >86%. In-vivo histological images at each site of probes, implanted by stitching using either glass capillary or ultrathin metal insertion needles exhibit seamless tissue-probe interfaces with negligible chronic immune response. In addition, electrophysiology studies demonstrated the ability to track single neuron activities at every injection site with chronic stability over at least one month. Notably, the measured spike amplitudes and signal-to-noise ratios at different implantation sites showed no statistically significant differences. Multi-site stitching implantation of flexible electronics in the brain opens up new opportunities for both fundamental neuroscience research and electrotherapeutic applications.