Freelance Mainframe Applications Developer Jobs in Florida

Citi is hiring a Senior Java Engineer to lead the technical transformation of its global cheque payment platform — a critical infrastructure at the heart of high-volume financial transaction processing. In this senior engineering role within the Payments Platform organisation, you will combine deep hands‑on coding in Java with platform strategy ownership, guiding a scrum team through the modernisation of core payment systems from legacy mainframe architectures to cloud-native microservices. This is a rare opportunity to architect and deliver at scale while embedding AI-driven practices into both the platform and the engineering workflow.

Responsibilities

• Design and build production-grade Java microservices for a high-volume, low-latency cheque payment platform, authoring reusable frameworks, conducting architecture reviews, and setting the technical standard for the team.

• Architect event-driven, fault-tolerant distributed systems using Apache Kafka — covering message schema design, topic partitioning, consumer group strategies, and exactly-once processing semantics.

• Define and execute the engineering roadmap for modernising the cheque payment platform, migrating global components from monolithic mainframe applications to scalable, cloud-native microservices.

• Own the platform's data architecture across Oracle, Microsoft SQL, and MongoDB — governing schema design, query optimisation, indexing strategies, and data consistency patterns across relational and NoSQL stores.

• Drive AI-powered engineering quality across the team — integrating tools such as GitHub Copilot and LLMs (including Gemini) for automated test generation, vulnerability scanning, and enhanced payment processing accuracy.

• Lead performance engineering for JVM-based applications — profiling hotspots, tuning garbage collection, optimising thread management, and improving critical settlement processing paths.

• Own the security and compliance posture of the platform through threat modelling, vulnerability assessments, and secure design reviews, ensuring alignment to engineering excellence standards across CI/CD, DevOps, and cloud-native delivery.

• Act as the primary engineering point of contact for the cheque processing pillar, communicating platform health, delivery progress, and engineering strategy to senior business and technology stakeholders.

Required qualifications & skills

• Extensive hands‑on Java engineering experience in high-throughput, production‑grade environments, including JVM performance tuning, GC optimisation, and thread and memory profiling.

• Proven ability to design and deliver microservices‑based architectures using Spring Boot or Spring MVC, with hands‑on expertise in RESTful API design, service decomposition, and inter‑service communication patterns.

• Deep practical knowledge of Apache Kafka, including topic design, partitioning strategies, consumer groups, exactly‑once semantics, and high-throughput stream processing.

• Hands‑on expertise with Oracle SQL and MongoDB, covering schema design, query optimisation, indexing, aggregation pipelines, sharding, and replica set configuration.

• Cloud‑native engineering experience with OpenShift, AWS, Kubernetes, and Docker — including container orchestration, autoscaling, and cloud-native deployment practices.

• Hands‑on ownership of CI/CD pipelines using tools such as Jenkins, Gradle, and Maven, with experience in DevOps practices, observability tooling such as Grafana, and distributed tracing.

• Demonstrated experience delivering at scale within Agile or SAFe frameworks, including PI planning, cross‑team dependency management, and programme‑level governance.

Beneficial skills & qualifications

• Familiarity with real‑time ML model serving, MLOps practices, RAG patterns, or agentic AI frameworks such as LangChain or AutoGen in an enterprise or regulated environment.

• Experience leading large‑scale legacy modernisation programmes — migrating mainframe components written in COBOL, Assembler, or C++ into cloud‑native microservices.

• Hands‑on Python skills applied to data pipeline engineering, AI and ML integration, and workflow automation.

• Knowledge of reconciliation and exception handling patterns within payment settlement workflows.

• Experience with chaos engineering, fault injection, and production readiness reviews as part of a resilience testing practice.

What we offer

Joining Citi means taking ownership of engineering that operates at genuine global scale. This role offers the platform, the people, and the technical challenge to do your most impactful work — with the seniority and visibility to shape how payments engineering evolves at one of the world's leading financial institutions.

• Technical leadership at scale, with end-to-end ownership of a mission‑critical global payment platform and clear senior visibility across technology and business.

• A hands‑on engineering culture that values technical depth at every level of seniority — you will stay close to the code while driving platform strategy.

• Access to cutting‑edge tooling, cloud‑native infrastructure, and AI‑powered development practices as part of a major payments modernisation programme.

• Hybrid working model with 3 days in the office and 2 days working remotely, offering flexibility alongside strong team collaboration.

• Structured learning and development opportunities to deepen expertise in distributed systems, AI integration, and financial technology at enterprise scale.

• Competitive financial wellbeing package, including salary, performance‑based reward, and a comprehensive suite of employee benefits.

• Access to wellbeing and family support programmes designed to help you perform at your best inside and outside of work.

Job Family Group:

Technology

Job Family:

Applications Development

Time Type:

Full time

Primary Location:

New Castle Delaware United States

Primary Location Full Time Salary Range:

$156,160.00 - $234,240.00

In addition to salary, Citi’s offerings may also include, for eligible employees, discretionary and formulaic incentive and retention awards. Citi offers competitive employee benefits, including: medical, dental & vision coverage; 401(k); life, accident, and disability insurance; and wellness programs. Citi also offers paid time off packages, including planned time off (vacation), unplanned time off (sick leave), and paid holidays. For additional information regarding Citi employee benefits, please visit citibenefits.com. Available offerings may vary by jurisdiction, job level, and date of hire.

Anticipated Posting Close Date:

Aug 14, 2026

Automated Processing and AI

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.

Illinois residents – AI Notice and Right

Citi is an equal opportunity employer, and qualified candidates will receive consideration without regard to their race, color, religion, sex, sexual orientation, gender identity, national origin, disability, status as a protected veteran, or any other characteristic protected by law.

If you are a person with a disability and need a reasonable accommodation to use our search tools and/or apply for a career opportunity review Accessibility at Citi. View Citi’s EEO Policy Statement and the Know Your Rights poster.