Employee mental health has devolved into a silent global crisis, exacting a significant and often unmeasured toll on economies worldwide. This paper transitions from a global conversation to local reality, analyzing the direct economic damage this challenge inflicts on Nigeria, arguing that neglecting workplace mental health is no longer a peripheral Human Resources (HR) concern but a pressing and quantifiable national economic threat. By synthesizing global data on productivity loss from absenteeism and presenteeism, this analysis contextualizes the staggering scale of the problem for the Nigerian economy. We further detail the direct economic drain from inflated healthcare expenditures and the immense, compounding costs of talent fragmentation, where skilled professionals opt out of the traditional workforce to become entrepreneurs, creators, or entertainers due to systemic burnout. Through detailed case studies from key Nigerian industries such as technology, banking, and healthcare, we illustrate the clear and compelling return on investment (ROI) for companies that move beyond reactive measures and proactively invest in comprehensive mental wellness infrastructure. This paper also provides a critical analysis of the existing Nigerian legal landscape and cultural barriers, arguing that foundational legislation like the Labour Act (Cap L1, Laws of the Federation of Nigeria 2004) is structurally insufficient, and outdated generational mindsets on mental health prevent meaningful progress. Given the compounding pressures of talent fragmentation away from the corporate sector and the rising prevalence of mental health conditions, continued inaction has become an unsustainable economic strategy for Nigeria. We conclude by recommending specific, evidence-based policy incentives and corporate practices designed to transform this significant economic liability into a powerful source of national competitive advantage.