Interfacial solute segregation behavior of P in nanocrystalline (NC) Ni alloys is investigated at the atomic scale using cross-correlative PED-APT measurements and atomistic simulations. Inhomogeneous P-segregation in grain boundaries (GBs) of NC Ni is observed from both experimental measurements and simulation calculations. Interfacial excess (IE) of the solute within GBs is further studied as function of GB misorientation. Significant scatter is detected in the IE of boundaries with similar misorientation angles and even within special boundaries with identical Σ values. From atomistic simulations, a general trend of increasing IE with initial GB energy and volume is observed but with poor correlation, indicating that the segregation behavior cannot be captured with a single average measure of the entire GB character. Rather the results suggest that the extent of interfacial solute segregation correlates better with fraction of high energy atomic sites in the boundary. By comparing IE values with thermodynamic model predictions, it is shown that the interfacial segregation behavior is strongly influenced by the energy distribution of atomic sites in the GBs. However, in order to predict the extent of solute segregation, the evolution of the atomic energy landscape in the GB with continuous solute segregation must be considered.