The strength of 9Cr steels, which is controlled by chemical composition and microstructure, evolves significantly under high temperature loading. This paper presents a temperature-independent, physically-based model for evolving yield strength, including the interdependent effects of dislocations, solutes, precipitates and grain boundaries. The key roles of solute and precipitate strengthening in 9Cr steels are successfully demonstrated. The measured significant beneficial effect of up to 3 wt% tungsten on solute strengthening, and hence, yield strength are successfully predicted. The new model demonstrates that the reported strength reduction in 9Cr-3W alloys under thermal aging can be primarily attributed to Laves phase formation and associated depletion of tungsten solutes, consistent with microstructural observations.