Understanding the thermal history of a metal part during additive manufacturing (AM) is essential for process design and microstructural control. In this study, detailed thermal history simulation was carried out for laser metal deposition (LMD) of Ti-6Al-4V (wt%) using the Directed Energy Deposition (DED) module in Simufact Welding. The simulation identified necessary LMD conditions for in-situ decomposition of α′-martensite in Ti-6Al-4V with respect to build height. On this basis, rectangular Ti-6Al-4V coupons were fabricated and systematic microstructural characterisation confirmed in-situ decomposition of α′ into ultrafine α-β lamellae in the as-built samples. In addition, the approximate in-situ transition time from α′ → α + β (lamellar) during the LMD process was determined to be 30–40 s, which is two orders of magnitude faster than conventional isothermal decomposition of α′. The underlying reasons were analysed and attributed to precursor (clusters) development within the α′-martensite laths as well as at the α′-lath boundaries due to being frequently and rapidly heated to temperatures well above the β transus, supported by recent literature. The as-built lamellar α-β Ti-6Al-4V achieved yield strength of 951 ± 10 MPa and tensile ductility of 8.18 ± 1.8%. Other insights obtained from this simulation-based experimental study were discussed including microstructural control of tall titanium alloy components through in-situ decomposition of α′-martensite.