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Ti–6Al–2Sn–4Zr–2Mo (Ti-6242), a near-$\alpha$ titanium alloy explicitly designed for high-temperature applications, consists of a martensitic structure after selective laser melting (SLM). However, martensite is thermally unstable and thus adverse to the long-term service at high temperatures. Hence, understanding martensite decomposition is a high priority for seeking post-heat treatment for SLMed Ti-6242. Besides, compared to the room-temperature titanium alloys like Ti–6Al–4V, aging treatment is indispensable to high-temperature near-$\alpha$ titanium alloys so that their microstructures and mechanical properties are pre-stabilized before working at elevated temperatures. Therefore, the aging response of the material is another concern of this study. To elaborate the two concerns, SLMed Ti-6242 was first isothermally annealed at 650${}^{\circ }\mathrm{\text{C}}\le T\le 1025^{\circ }$C and then water-quenched to room temperature, followed by standard aging at 595${}^{\circ }$C. The microstructure analysis revealed a temperature-dependent martensite decomposition, which proceeded sluggishly at $T\le 700^{\circ }$C despite a long duration but rapidly transformed into lamellar $\alpha +\beta$ above the martensite transition zone (770${}^{\circ }\mathrm{\text{C}}\le T\le 800^{\circ }$C). As heating to $T>\beta -\mathrm{\text{transus}}(993^{\circ }$C), it produced a coarse microstructure containing new martensites formed in water quenching. The subsequent mechanical testing indicated that SLM-built Ti-6242 is excellent in terms of both room- and high-temperature tensile properties, with around 1400 MPa (UTS)$+$5% elongation and 1150 MPa (UTS)$+$10% elongation, respectively. However, the combination of water quenching and aging embrittled the as-built material severely.