Individuals successful at weight loss maintenance (WTLM) utilize similar behaviors but in varying amounts and combinations. Research identifying characteristics which predict WTLM success could be used to develop effective WTLM treatment programs. The purpose of this retrospective analysis was to determine behavioral (e.g., self monitoring of dietary intake) and biological factors (e.g., sex) which may influence WTLM success. Methods: In a previously conducted 12-month WTLM study, weight-reduced middle-aged and older men and women (n = 39) were assigned behavioral goals and asked to daily self-monitor body weight, fruit and vegetable intake, water consumption, and physical activity. This investigation utilized a crisp set qualitative comparative analysis (QCA) to characterize individuals’ weight changes and behaviors. Sex difference in clinically significant weight loss (WL) (≥5% WL) at 12 months was investigated, and growth curve models estimated interaction effects between sex and WTLM predictors. Results: QCA findings suggest patterns of weight change during the initial three months of WTLM treatment may dictate 12-month weight outcomes. No sex difference was found in clinically significant WL or in the interaction of sex and behaviors on weight change. Conclusions: WTLM treatments should provide more intensive support during transition from WL to WTLM, particularly during the first three months.