In-process laser-based heat treatments (LHTs) during additive manufacturing are explored as an alternative to post-process heat treatments such as annealing, to alter the microstructure and mechanical properties of Ti-6Al-4V. Three samples were manufactured via laser-based direct energy deposition with the laser operating at 300 W. One as-built sample without any LHTs served as the reference sample. For the remaining two, each deposited layer was subjected to an additional LHT just after its deposition; LHTs were performed at 100 W for one (LHT100) and 300 W for the other (LHT300) without powder addition. LHT100 showed an overall improvement in the strength/ductility trade-off. Whereas LHT300 resulted in a lower strength but a much higher ductility and toughness than the other two samples. Synchrotron X-ray diffraction analysis of as-built samples revealed an insignificant difference in the bcc (β) and combined hcp (martensite alpha', diffusively formed alpha_d and massively transformed alpha_m) phases between different samples. However, scanning electron microscopy uncovered a significant effect of LHTs in varying the fractions of different hcp phases between samples, which is the main cause behind mechanical property differences. These results demonstrate the significant potential of in-process LHTs to tailor alloy microstructures for achieving desired mechanical properties.