Mainframe System Programmer Jobs in Ontario (NOW HIRING)

Citi is looking for a Senior Java Developer to join its Applications Development team, where you will design, build, and own high-performance Java applications that underpin critical financial services at global scale. This is a hands-on technical role where your expertise will directly shape the quality, architecture, and reliability of systems that process high-volume, latency-sensitive workloads. If you are an engineer who takes full ownership of what you build and thrives in a fast-moving, technically demanding environment, this is the role for you.

• Design and develop efficient, reusable, and reliable Java code for high-volume, low-latency applications, setting the benchmark for engineering quality across the team.

• Architect RESTful APIs and scalable backend services using Spring and Spring Boot, ensuring optimal performance, responsiveness, and maintainability from day one.

• Translate complex business and user requirements into detailed technical specifications, low-level design documents, and solution architecture artefacts.

• Identify, evaluate, and champion the adoption of emerging technologies and frameworks that meaningfully enhance the application stack.

• Build and maintain comprehensive testing automation — covering unit, integration, and performance tests — to uphold code quality and production reliability.

• Lead peer code reviews, providing precise and constructive feedback that raises engineering standards across the team.

• Take full accountability for production systems, including rapid diagnosis and resolution of critical issues to minimise customer and business impact.

• Partner with business analysts, project managers, and senior engineers across functions to deliver high-quality software, while establishing and maintaining clear standards for technical documentation.

• 5 or more years of professional Java development experience, with substantial exposure to cloud-based environments and distributed systems.

• Expert-level command of Java, RESTful API design, and cloud engineering principles, with hands-on experience building production-grade services using Spring and Spring Boot.

• Solid database experience spanning relational systems such as Oracle and SQL, as well as NoSQL technologies including MongoDB.

• Practical experience with cloud-native platforms including OpenShift and Kubernetes, and messaging technologies such as MQ and Kafka.

• Deep commitment to clean code principles, design patterns, and test-driven development practices that produce maintainable, production-ready software.

• Fluency in Agile methodologies and the full software development lifecycle, with hands-on experience building and maintaining CI/CD pipelines using DevOps tooling.

• Demonstrated ability to lead technical delivery autonomously, manage competing priorities, and perform effectively under pressure in a fast-paced environment.

• Background in financial services with a focus on payments processing, giving you immediate context for the systems you will be building.

• Familiarity with Generative AI and large language models, and an interest in applying these technologies within an enterprise engineering context.

• Experience designing integration patterns between mainframe systems and modern event-driven or API-based architectures using Kafka.

• Recognised depth of expertise as a subject matter expert in at least one critical area of applications development.

At Citi, you will work on systems that matter — at a scale few organisations can match. Our Applications Development teams operate at the intersection of engineering excellence and real-world financial impact, giving you the technical complexity, cross-functional collaboration, and career depth to grow as an engineer and leader. We support your professional and personal wellbeing with a benefits package designed to deliver long-term value.

• Hybrid working model with 3 days in the office and 2 days working remotely, giving you the structure of in-person collaboration and the flexibility of remote work.

• Exposure to globally significant, technically complex systems across one of the world's leading financial institutions.

• Ongoing learning and development opportunities, including access to technical training, architectural review processes, and cross-team knowledge sharing.

• A performance-driven environment where engineers are empowered to make meaningful technical decisions and take ownership of outcomes.

• Competitive compensation and financial wellbeing support, including access to Citi's broader benefits programme tailored to your location.

• Wellbeing and work-life balance support, with resources designed to sustain long-term performance and personal fulfilment.

Apply now to bring your Java engineering expertise to Citi and help build the high-performance systems that power global financial services.

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We use automated processing, including artificial intelligence, for our legitimate business interests (or our reasonable and appropriate business purposes) to identify and align the candidate's skills and abilities with a specific job opening. Additionally, if you so choose, or consent, we can match your skills and abilities to other suitable roles at Citi.

Importantly, all our hiring processes and decisions, including determining your suitability for a role, are conducted, checked, and decided by individuals. Our automated processing and AI do not involve relying on automatic or autonomous decision-making. Please refer to any Jurisdictional Considerations, with specific provisions for your country (where relevant) for further details.

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This job opening is for an existing job vacancy.

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