Ensuring the structural reliability of composite materials requires a thorough understanding of their performance under various service conditions, particularly in demanding aerospace environments. This study investigates the influence of low-temperature (LT), high-temperature (HT), and cyclic-hygrothermal (CHT) conditions—representing a broad spectrum of realistic operational environments—on the out-of-plane and in-plane mechanical properties and damage behavior of unidirectional carbon fiber/poly-ether-ketone-ketone (CF/PEKK) composites. To assess the impact of these environmental factors on mechanical performance and failure mechanisms, a comprehensive experimental approach is employed, incorporating double-cantilever beam (DCB) tests with acoustic emission (AE) monitoring, end-notched flexure (ENF), short beam shear (SBS), and three-point bending (3-PB), alongside microscopic analysis. Results emphasize the significant influence of environmental conditions on the mechanical performance and damage evolution of CF/PEKK composites. Under LT conditions, flexural strength and deflection improve due to enhanced interlaminar interactions, despite increased brittleness in the matrix. LT conditioning yields the highest interlaminar shear strength (ILSS) and damage nucleation energy. However, LT-conditioned specimens exhibit brittle fractures with unstable crack propagation and frequent fiber-tow breakage during DCB and ENF tests. Conversely, HT conditions reduce flexural strength, but crack onset is delayed because of increased ductility. HT conditioning decreases ILSS and stiffness due to thermal softening. CHT conditioning results in intermediate flexural strength due to matrix plasticization, yet induces substantial inelastic deformation, multiple delaminations, and reduced interlaminar fracture toughness. These novel findings highlight the critical role of environmental factors in designing thermoplastic-based composites for aerospace applications, emphasizing the need for optimization to ensure reliable performance under diverse conditions.