Obesity is a multifactorial disease characterized by an excessive and abnormal accumulation of body fat that results from both genetic and environmental factors. In this review, we revisited the literature on the variability of obesity-associated genes and their impact on the effectiveness of obesity treatment interventions. Individuals harboring variants of these genes were found to have either better or worse outcomes after weight loss therapies. The majority of the genetic variants were identified in genes that play a role in the leptin-melanocortin pathway (LEPR, NPY, POMC, MC4R, GHRL, GHSR, GLP-1R, BDNF), which regulates food intake and energy expenditure. Both these processes are key elements for energy homeostasis, therefore relevant for the success/failure of weight loss strategies. Some genetic alterations were found to modulate the outcomes of different weight loss interventions, while others were only linked to the effectiveness of bariatric surgery, according to the studies here included and available. Herein, we revisited the most relevant molecular data, with a primarily focus on human studies, concerning how the genetic background influences the outcomes of weight loss interventions. Our aim is to gather relevant information on the genetic data related to weight loss strategies that can be compelling to guide clinical decisions, setting realistic expectations, and ultimately improving the long-term health conditions of individuals with obesity.