Natural antioxidants for the shelf-life prolongation of fat, oil and pet food are of great interest. Beef tallow is a rendered fat from cattle that is usually added to pet food, offering numerous health benefits and making meals more appetizing, especially for dogs. It is rich in essential fatty acids, vitamins A, D, E, and K, which are important for a dog’s health. Loxidan E Ros Liquid + is a very efficient natural antioxidant blend including tocopherol extract and rosemary extract. The objective of this study was to investigate whether the addition of 300 ppm Loxidan E Ros Liquid + into beef tallow can prolong its shelf-life. Additionally, it also aimed to examine the effect of natural antioxidants on the shelf-life of a finished pet food, which contains Loxidan E Ros Liquid +. All three samples, including unstabilized beef tallow, beef tallow stabilized with 300 ppm Loxidan E Ros Liquid +, and a finished pet food stabilized with Loxidan E Ros Liquid +, were subjected to the accelerated storage trial at 41 °C. The storage trial was conducted for 12 weeks for both samples of beef tallow and for 18 weeks for the finished pet food. After regular intervals, the peroxide value (POV) and anisidine value (AnV) were measured to track the oxidative status of the samples. At 41 °C, a storage time of 1 week roughly translates to a storage time of 1 month at room temperature. Results showed that the POV and AnV of the unstabilized beef tallow increased significantly during the storage for 12 weeks at 41 °C, while the POV and AnV for the stabilized sample remained low during this storage time. This indicates a good efficacy of Loxidan E Ros Liquid + in the tested beef tallow. The POV of the tested pet food remained low at values well below 10 meqO2/kg for the first 12 weeks. After 18 weeks, the POV was at a value of approximately 22 meqO2/kg, indicating a significant acceleration of the oxidation after 12–18 weeks. The AnV remained low at values <0.5 during the storage trial. Adapted to room temperature, a shelf-life of 12 months, but probably less than 18 months can be estimated. A further addition of Loxidan E Ros Liquid + could be considered, if longer shelf-life is needed.