We conducted a mixed methods study in an upper-level genetics course exploring the relationships between student motivation, homework completion, and academic achievement at the college level. We used data from an open-ended questionnaire, homework grades and completion reports, and exam scores. We used these data sources to measure self-perceived motivating/demotivating factors and then tested these factors for correlation with homework completion and academic achievement. We found no significance in homework completion when considering credit or extra credit as a motivating factor. According to student reports they completed significantly more homework when considering reinforcement of content as a motivating factor. However, we found discrepancies between students' reported motivation and actual completion rates. Self-reported study style, self-perceived conscientiousness, intelligence, attitude, time commitments, and complexity of assignments had significant impacts on whether or not students completed homework assignments and impacted students' academic achievement. Overall, we found a positive relationship between homework completion and academic achievement within this upper-level college genetics course and provide implications for increasing student motivation.