With the completion of proton--proton collision data-taking in 2024, the ATLAS experiment has recorded an integrated luminosity exceeding 180~fb$^{-1}$ during Run~3, which began in July 2022 following the Long Shutdown~2 (LS2).
During LS2, several detector upgrades were implemented, most notably the installation of the New Small Wheel (NSW), which replaced the innermost stations of the Muon Spectrometer endcaps.
The ATLAS Muon Spectrometer, the largest muon system ever built for a collider experiment, now combines long-standing gaseous detectors—Monitored Drift Tubes (MDT), Thin Gap Chambers (TGC), and Resistive Plate Chambers (RPC)—operational for over 15~years, with newer technologies such as Micromegas and small-strip TGCs introduced with the NSW.
These new systems are now fully operational after an extensive construction and commissioning phase, enhancing both muon tracking and trigger performance.
This contribution presents the performance of the ATLAS Muon Spectrometer, mentioning the long-term stability of the legacy detectors and focusing on the NSW upgrade, including its impact on simulation, alignment, track reconstruction, and trigger performance.