Within Quenching and Partitioning (Q&P) processes, isothermal treatments following partial formation of martensite are now of great interest for the industrial sector since the formation of bainitic ferrite is accelerated by the presence of even small fractions of athermal martensite. This acceleration effect can lead to faster and less energy consumption processing routes for manufacturing bainitic steels in comparison with conventional heat treatments above Ms. For a feasible industrial implementation, it is however necessary to determine the mechanical behaviour of the steels made under those conditions. The contribution of the combination of prior athermal martensite and bainitic ferrite to the mechanical properties should be considered. This study focuses on the effect of small fractions of athermal martensite formed prior to isothermal treatments on the mechanical properties, confronting with microstructures made by isothermal treatments without prior formation of martensite. Dog-bone specimens of a low-C high-Si steel were heat-treated by dilatometry at different temperatures above and below Ms during one hour. Afterwards, microhardness measurements and tensile tests were performed in order to characterise the mechanical properties of the mentioned specimens, such as yield strength, ultimate tensile strength, and strain hardening. Differences in mechanical properties were observed depending on the product phases formed from the decomposition of austenite in treatments above and below Ms. In particular, small fractions of athermal martensite in treatments below Ms led to higher yield strength than those ones treated above Ms.