Pain and distress are widespread for people with dementia. However, effective pain management is limited by the quality of assessment tools. In this study, the development and trial of the Mahoney Pain Scale, which aims to assess pain in advanced dementia and distinguish it from agitation is described. A total of 112 participants with advanced dementia who experienced either pain, agitation, neither or both were assessed via the Mahoney Pain Scale during a pleasant and aversive activity. The Mahoney Pain Scale demonstrated adequate interrater reliability and internal consistency. As predicted, participants experiencing pain and/or agitation obtained higher Mahoney Pain Scale scores during the aversive activity. Participants also differed with respect to their pattern of scores, and consequently, the Mahoney Pain Scale differentiated pain states from non-pain ones. The clinical impressions of nurses who trialed the tool were favorable; they reported that it seemed accurate and easy to use. Thus, the Mahoney Pain Scale may be useful for assessing pain in dementia.