Objective To identify diagnostic groups and the form of the trajectory of functional decline that has the potential to enhance advance care planning (ACP) in a nursing home (NH) population. Methods Retrospective, longitudinal study with dependent variable (function) derived from the Resident Classification Scale (RCS), 1997–2008. Trajectory modelling used linear and curvilinear terms. Results The organ failure or other residents have a linear average functional decline. The organ failure residents had an average decline of 6.5 points per year and average function score at death of 68.6, CI [62.4, 74.8]. The cancer and frailty residents had significant curvilinear terms. The frailty residents had a slower rate of decline at 9.54 points per year and were most care dependent at death, with an average function score of 77.1, CI [73.8, 80.9]. Conclusion Functional change is a measurable variable for a predictive tool to enhance ACP for NH residents based on their diagnosis.