Sr. Application Developer - Legacy System Modernization

The Senior Application Developer – Legacy Modernization strengthens and evolves legacy systems by improving operational stability, enhancing observability, and migrating core services to modern, scalable platforms. This role designs and executes modernization strategies, establishes engineering standards, and leverages automation, CI/CD, and AI-assisted development tools to accelerate system transformation. It balances innovation with business continuity by improving system reliability, enabling efficient incident response, and managing technical risk across both legacy and modern environments.

Principal Responsibilities And Duties

• Designs, develops, and implements the incremental migration of legacy application logic to C# / .Net, establishing scalable patterns, reusable components, and engineering standards.
• Evaluates existing batch and online processes for modernization readiness and defines migration strategies that balance technical risk, cost, and business continuity.
• Establishes and maintains CI/CD pipelines, integrates automated testing into build processes, and defines code review and branching standards.
• Leverages generative and agentic AI tools to accelerate code analysis, refactoring, migration, and test generation across both legacy and modern codebases.
• Develops modular, well-documented, and testable code with strong unit and integration test coverage to support long-term maintainability.
• Conducts thorough code reviews focused on correctness, readability, performance, and adherence to established patterns, incorporating AI-assisted review tooling where appropriate.
• Identifies inefficiencies in systems and workflows, proposing practical, data-driven improvements that enhance reliability, performance, and productivity.
• Troubleshoots and resolves issues in legacy and modern systems by triaging incidents, performing root cause analysis, and driving timely resolution based on business impact.
• Enhances system instrumentation to increase observability and lower the long-term cost of maintenance, evaluating AI-assisted monitoring and anomaly detection where appropriate.
• Identifies, documents, and manages operational and technical risks, prioritizing resolution based on business impact.
• Coordinates cross-functionally during active incidents, communicating status and resolution progress to relevant stakeholders and escalating appropriately when issues exceed defined scope or severity thresholds.
• Contributes to the growth of operational knowledge artifacts by authoring and maintaining runbooks, post-mortems, and internal documentation – translating incident learnings into reusable institutional knowledge that reduces future resolution time.
• Other duties as needed or assigned.

Skills And Abilities Required

• Advanced understanding of information processing principles and practices.
• Expert-level proficiency in C# and .Net.
• Experience with microservices architectures; ability to select appropriate tools and frameworks based on system requirements.
• Solid understanding of relational databases and SQL, including performance optimization and schema design.
• Demonstrated ability to work in AI-augmented development environments, leveraging generative and agentic AI tools for design, code generation, and automation.
• Experience with incremental decomposition of monolithic systems into domain services utilizing patterns such as strangler fig or branch by abstraction.
• Experience building and maintaining CI/CD pipelines (GitHub Actions, Azure DevOps, or similar), integrating test automation within the build pipeline, and managing infrastructure or deployments through code.
• Experience leveraging agentic AI systems to assist with development tasks, with strong validation, testing, and quality assurance practices.
• Working knowledge of data privacy, security, and regulatory considerations in application development.
• Advanced analytical, conceptual, and problem-solving abilities.
• Strong understanding of the organizational goals and alignment of technical solutions to business outcomes.
• Effective written and oral communication skills, including the ability to explain complex technical concepts to non-technical stakeholders.
• Strong interpersonal and consultative skills, with the ability to collaborate across technical and business teams.
• Ability to effectively prioritize and execute tasks in a high-pressure environment.
• Experience working in cross-functional, agile teams required.

Minimum Level Of Preparation And Training Normally Required

• Bachelor’s degree in the field of computer science, information systems, or computer engineering or equivalent experience.
• 5+ years of application development experience across the full software development lifecycle (design, build, test, and delivery).
• 5+ years of experience with business requirements analysis and translation into technical solutions.
• 3+ years of hands-on experience with Microsoft development tools, including .NET, Azure DevOps, and GitHub.
• 3+ years of hands-on experience with relational database technology, preferably Microsoft SQL Server.
• Experience in payment processing or financial service domains preferred.
• HP NonStop experience strongly preferred; candidates with strong legacy expertise and demonstrated ability to adapt to modern platforms will be considered.

Application Process

You can directly apply through Cass's website at https://www.cassinfo.com/careers. Please apply directly to this position via the “Apply” button. You will be required to create an account and provide your resume, contact information and other pertinent employment information. This process typically takes 20 minutes or less. Should we find that you meet the minimum requirement of the position, a member of our recruiting team will be in touch to start the interview process.

About Our Company

Cass Information Systems, Inc. (NASDAQ: CASS) is a leading provider of integrated information and payment management solutions. Cass enables enterprises to achieve visibility, control and efficiency in their supply chains, communication networks, facilities, and other operations. Disbursing over $94 billion annually on behalf of its clients, and with total assets of $2.5 billion, Cass is uniquely supported by Cass Commercial Bank. Founded in 1906 and a wholly-owned subsidiary, Cass Commercial Bank provides sophisticated financial exchange services to the parent organization and its clients. Cass is part of the Russell 2000®. More information is available at www.cassinfo.com.