Preservatives are essential for ensuring the stability, safety, and efficacy of pharmaceuticals, cosmetics, and food products. However, synthetic preservatives often raise toxicity concerns. This study evaluated Rosmarinus officinalis (rosemary) leaf extracts and coffee by-products from Coffea arabica and Coffea canephora as potential natural preservatives for emulsions. Antimicrobial activity was assessed against Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans, along with cytotoxicity tests on human keratinocytes and antioxidant activity. The most effective extracts were incorporated into an oil-in-water emulsion for evaluation. C. arabica extracts showed the best results among coffee samples, with 43.53 mg GAE/g (gallic acid equivalents) and 2.32 mg QE/g of total phenolics (quercetin equivalents) and flavonoids, and minimum inhibitory concentrations (MICs) of 12.5 mg/mL against Escherichia coli, and 25 mg/mL against Staphylococcus aureus and Pseudomonas aeruginosa. Rosemary extract showed 158.01 ± 23.67 mg GAE/g and 1.95 ± 0.05 mg QE/g, with MICs of 2.5 mg/mL against E. coli, 1.25 mg/mL against P. aeruginosa, 0.3 mg/mL against S. aureus, and 0.08 mg/mL against Candida albicans. However, rosemary extracts displayed complete inhibition of keratinocyte growth at 20 µg/mL. A combination of both extracts had synergistic effects against S. aureus and P. aeruginosa. The emulsion met microbial safety standards in the challenge test for bacteria but not yeast. The results suggest that rosemary extracts enhance the potential of coffee by-product as a preservative system, and as a multifunctional excipient system in cosmetics, offering preservation and antioxidant protection. However, further strategies, such as adding other ingredients or adjusting the formulation pH, are required to ensure yeast inhibition.