Effective weight management interventions involve a combination of behavioral strategies focusing on dietary changes. Tracing the change through mobile apps has been proven to be a valuable platform for facilitating weight management in many countries. However, the effectiveness of mobile app-based dietary intervention on weight management in Taiwan remains to be determined. By using the designated mobile app, this study aimed to assess the efficacy of the diet intervention, which is based on a 2:1:1 portion control plate and a flexible low-carbohydrate (FLC) diet. This 8-week retrospective cohort study involved 10,297 participants who were divided into two groups: the intervention group (joined an 8-week diet intervention program with the daily diet record assessed by registered dietitians) and the control group (voluntarily using the app without instructional materials or coaching). After eight weeks of intervention, the intervention group showed a higher weight loss percentage (−4.78% vs. −1.54%), body mass index (BMI) (−1.26 kg/m² vs. 0.69 kg/m²), and diet record completeness (73.52% vs. 28.91%) compared with the control group. With respect to gender, male participants showed higher baseline weight and higher weight loss (−6.02%) in the intervention group. In the intervention group, 2871 participants (33.4%) lost less than 4% of their weight, 5071 participants (58.9%) lost 4–8% of their body weight, and 662 participants (7.7%) lost >8% of their weight. Compared to the low-effectiveness group (weight lost 8%) had a significantly higher diet record completeness (91.61 ± 15.99 vs. 55.81 ± 32.92), dietary compliance (green light %) (88.93 ± 9.9 vs. 77.75 ±17.5), protein intake % (26.34 ± 2.85 vs. 23.49 ± 3.56), and fat intake % (49.66 ± 6.36 vs. 44.05 ± 7.37). Most importantly, the high-effectiveness group had a lower carbohydrate intake % (24.1 ± 7.86 vs. 32.46 ± 9.61). The results remained significant after being stratified by gender. This study found that the use of online applications plus the intervention of dietitians is beneficial for short-term weight loss. The composition of nutrients and dietary compliance also significantly impacted weight loss.