This study examines the role of business accelerators in fostering startup performance across diverse entrepreneurial ecosystems, focusing on Romania, the United States, and Italy. Using a cross-country comparative approach, this research investigates how accelerators influence grant utilization efficiency, financial management capabilities, sustainable business practices, and income growth and diversification. The findings reveal that accelerators significantly enhance startups’ financial resource management and revenue growth, with variations across the target countries. In Romania, accelerators address structural barriers and resource constraints, while in the United States, they drive sector-specific innovations within a mature ecosystem. Italy highlights a blend of traditional and emerging industries, with accelerators fostering sustainability initiatives and innovation. This study introduces sustainable business practices and financial management capabilities as mediating factors, developing existing theoretical models. Practical implications for entrepreneurs, policymakers, and investors are outlined, emphasizing personalized business accelerator strategies to address contextual challenges.