Tek P. Adhikari CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, China School of Astronomy and Space Science, University of Science and Technology of China, Hefei, China, Gopal Bhatta Janusz Gil Institute of Astronomy, University of Zielona Góra, Zielona Góra, Poland, Sangeetha Kizhakkekalam Janusz Gil Institute of Astronomy, University of Zielona Góra, Zielona Góra, Poland Doctoral School of Exact and Technical Sciences, University of Zielona Góra, Zielona Góra, Poland, Navaneeth P K Janusz Gil Institute of Astronomy, University of Zielona Góra, Zielona Góra, Poland Doctoral School of Exact and Technical Sciences, University of Zielona Góra, Zielona Góra, Poland, Alex Markowitz Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Warsaw, Poland, Zhicheng He CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, China School of Astronomy and Space Science, University of Science and Technology of China, Hefei, China, Santanu Mondal Department of Physics and Electronics, Christ University, Bangalore, India

We present a long-term X-ray spectral and timing analysis of the TeV blazar Mrk~421 based on nearly a quarter century of archival \textit{XMM-Newton}/pn observations obtained between 2000 and 2025. Our sample comprises 76 epochs obtained with the pn instrument in both IMAGING and TIMING modes, allowing us to trace the source's variability and spectral evolution. Mrk~421 exhibits flux variations by approximately an order of magnitude, with prominent flaring periods interspersed with low-activity states. A clear harder-when-brighter trend is observed, accompanied by scatter in the hardness ratio (HR), suggesting flux alone does not uniquely determine the spectral state. Fractional variability in the hard band ($2-10$ keV) correlates tightly with that in the soft band ($0.3-2$ keV), with a steeper slope indicating pronounced variability at higher energies. The flux distribution is non-Gaussian and lognormal-like, dominated by moderate flux levels with a tail of bright flares. Spectral analysis shows that power-law model is not sufficient; 59 epochs are best described by a broken power-law (BPL) and 17 by a log-parabola (LP). This fluctuation between BPL and LP models suggests the emission region transitions between a distinct acceleration shockfront and downstream stochastic turbulence. The spectral break energy of BPL remains predominantly clustered around $2$ keV across all flux states, while the average spectral steepening above the break is mild ($ΔΓ\approx 0.19$), inconsistent with standard one-zone cooling models. Our results support structured jet scenarios or energy-dependent electron escape, providing key constraints on the long-term behavior of relativistic jets in high-synchrotron-peaked blazars.