Controlling the crystal phase in layered transition metal dichalcogenides (TMD) is critical for their diverse, flexible applications. However, due to the thermodynamic stability of 2H over other polymorphs, fine synthesis control over polymorphism in TMD is challenging, restricting the entire range of characteristics associated with other polymorphs. Herein, we present a solution-based crystal phase engineering approach for layered transition metal disulphide nanosheets by modulating the reactivity of the molecular precursors. By tuning precursor-ligand chemistry, 2H, 1T’ and polytypic MoS2 and WS2 are synthesised. The flexibility to selectively modify the reactivity of S and metal precursors allowed control over the proportion of specific phases in synthesised nanosheets. The formation of 1T’ is facilitated by the highly reactive metal and S source, whereas less reactive sources lead to the formation of thermodynamically stable 2H. We provide in-depth mechanistic insights that enable improved predictive synthesis of particular phases. In addition, we carried out a retrosynthetic approach that predicted chemical intermediates during the nanosheet formation, leading to the specific polymorph formation. Furthermore, we demonstrate the advantage of the polytypic transition metal sulfide nanosheets over the monophasic nanosheets displaying better catalytic performance with higher onset potentials and diffusion-limited current density during the oxygen reduction reaction (ORR). Our synthesis approach has the potential to be extended to various TMDs compositions, enabling exquisite control over polymorphism in TMDs.