This paper introduces the concept of Markov length as a key quantity for characterizing mixed-state quantum phases and transitions. The Markov length is defined as the length scale at which the quantum conditional mutual information (CMI) decays exponentially. For a state evolving under a local Lindbladian, if its Markov length remains finite, the state remains in the same phase, implying the existence of a quasi-local Lindbladian evolution that can reverse the former. The authors apply this diagnostic to the toric code subject to decoherence, showing that the Markov length is finite everywhere except at the decodability transition, where it diverges. The CMI in this case can be mapped to the free energy cost of point defects in the random bond Ising model, implying that the mixed state phase transition coincides with the decodability transition and suggests a quasi-local decoding channel. The paper also discusses the role of Markov length in understanding the stability of mixed-state phases under perturbations, and its connection to the correlation length in ground state phases. The results demonstrate that the Markov length provides a criterion for the stability of mixed-state phases, similar to the energy gap in pure state phases. The study highlights the importance of Markov length in characterizing mixed-state quantum phases and transitions, and its potential applications in quantum error correction and quantum information theory.This paper introduces the concept of Markov length as a key quantity for characterizing mixed-state quantum phases and transitions. The Markov length is defined as the length scale at which the quantum conditional mutual information (CMI) decays exponentially. For a state evolving under a local Lindbladian, if its Markov length remains finite, the state remains in the same phase, implying the existence of a quasi-local Lindbladian evolution that can reverse the former. The authors apply this diagnostic to the toric code subject to decoherence, showing that the Markov length is finite everywhere except at the decodability transition, where it diverges. The CMI in this case can be mapped to the free energy cost of point defects in the random bond Ising model, implying that the mixed state phase transition coincides with the decodability transition and suggests a quasi-local decoding channel. The paper also discusses the role of Markov length in understanding the stability of mixed-state phases under perturbations, and its connection to the correlation length in ground state phases. The results demonstrate that the Markov length provides a criterion for the stability of mixed-state phases, similar to the energy gap in pure state phases. The study highlights the importance of Markov length in characterizing mixed-state quantum phases and transitions, and its potential applications in quantum error correction and quantum information theory.