This work reports the measurement of electron g-factor anisotropy (|Δg|=|g001g1¯10|) for phosphorous donor qubits in strained silicon (sSi = Si/Si1xGex) environments. Multimillion-atom tight-binding simulations are performed to understand the measured decrease in |Δg| as a function of x, which is attributed to a reduction in the interface-related anisotropy. For x<7%, the variation in |Δg| is linear and can be described by ηxx, where ηx1.62×103. At x=20%, the measured |Δg| is 1.2±0.04×103, which is in good agreement with the computed value of 1×103. When strain and electric fields are applied simultaneously, the strain effect is predicted to play a dominant role on |Δg|. Our results provide useful insights on the spin properties of sSi:P for spin qubits, and more generally for devices in spintronics and valleytronics areas of research.