The Senior Staff Software Systems Engineer serves as a senior technical authority across one or more mission-system domains supporting United States Government customers. This role is responsible for defining, integrating, and evolving complex software-intensive mission systems spanning software, cloud infrastructure, networking, cybersecurity, data management, and operational workflows. Working across engineering disciplines and organizational boundaries, this engineer ensures independently developed capabilities function as a cohesive, resilient, secure, and mission-ready system. The role partners directly with government customers, program leadership, domain owners, architects, software engineers, operations, and security stakeholders to understand mission needs, shape technical direction, resolve cross-domain issues, and guide execution across multiple engineering teams. Success in this role requires strong systems thinking, software architecture expertise, technical leadership, and the ability to balance mission performance, interoperability, cybersecurity, schedule, lifecycle cost, and long-term maintainability across complex operational environments. This position involves regular interaction with customers supporting classified and unclassified government programs and may require limited travel.