In this study, researchers analyzed the reliability and validity of the mentor's assessment for principal internships at a university in the Southeast region of the United States. The results of the study yielded how trustworthy and dependable the instrument is and the effectiveness of the instrument in the current principal preparation program. Study results were analyzed using reliability requirements for instruments utilized in obtaining national accreditation and to provide suggestions for program improvement. The instrument was reviewed by a panel of experts from areas outside of the research population to establish content and face validity. Internal consistency and reliability were measured using Cronbach's alpha. A total of 229 candidate internship scores were used. The reliability test resulted in an overall alpha of 0.949. The results obtained in this study indicate the instrument has a very high level of validity as well as reliability.