Senior Software Engineer

NMI is a leading provider of payment gateway solutions, specialising in card-present solutions and payment processing. We are seeking a Senior Software Engineer to join our Protocols team in the UK. The Protocols team owns the services that manage connectivity between the NMI platform and external card acquirers and payment processors. These mission-critical systems, built primarily in C#, power real-time transaction processing and must operate with high levels of reliability, security, scalability, and observability.

As part of the wider Payments Gateway group, the Protocols team plays a central role in ensuring seamless payment flow across our platform. As a P3 Senior Engineer, you will take ownership of complex components within this domain, contribute to architectural decisions, and guide the delivery of robust, high-performance solutions. This role is suited to engineers who operate with strong technical judgement, thrive in high-availability environments, and are motivated to continuously improve the resilience and maintainability of business-critical systems.

The ideal candidate is a highly skilled software engineer with senior-level technical depth, strong system design experience, and the curiosity to work across components when needed.

As a Senior Engineer, you will take ownership of complex technical challenges and play a key role in designing, developing, and evolving backend services that enable secure, performant, and reliable payment transactions. You will influence architectural decisions, proactively identify risks and technical debt, and contribute to strengthening our CI/CD, testing, and deployment practices.

You are comfortable operating in high-availability environments where reliability and security are paramount. You lead by example through high engineering standards, thoughtful design, and mentorship of other engineers.

Key Responsibilities

• Design and deliver complex backend systems that power secure, scalable, and high-performance payment processing.
• Take ownership of significant components or services within the Core Gateway platform, ensuring long-term maintainability and operational excellence.
• Clearly articulate complex technical solutions and trade-offs to engineers, Product, and stakeholders.
• Guide project delivery within the team, helping break down work, manage technical risk, and ensure high-quality outcomes.
• Proactively identify and resolve architectural gaps, performance bottlenecks, and reliability risks.
• Lead and participate in design reviews, raising the quality bar across the team.
• Drive improvements in observability, fault tolerance, and operational readiness across services.
• Mentor junior and mid-level engineers, providing constructive feedback and supporting their technical growth.
• Contribute to the evolution of CI/CD pipelines, testing strategies, and deployment automation.
• Ensure adherence to strong security and compliance standards (e.g., PCI, OWASP, SOC2).
• Participate in the team’s out-of-hours on-call rota, contributing to incident response, root cause analysis, and preventative improvements (typically after a 6-month onboarding period, with additional compensation provided).

Skills & Experience

• Bachelor's degree in Computer Science, Information Technology, or equivalent experience.
• Minimum of 5 years of experience developing complex software applications in a commercial environment.
• Advanced proficiency in C# (strongly preferred) or Java, with strong understanding of object-oriented design principles.
• Deep understanding of the full software development lifecycle (SDLC) and agile methodologies.
• Experience with relational databases (e.g., MySQL, SQL Server, Oracle).
• Strong knowledge of security best practices (e.g. OWASP, PCI, SOC2, HIPAA).
• Proficiency with Google Cloud Platform (GCP), Amazon Web Services (AWS), or similar cloud environments.
• Experience with logging systems (such as Logz.io) and optimizing logs preferred.
• Experience with monitoring and alerting systems (e.g. DataDog, NewRelic, etc.) preferred.
• Experience designing and building scalable, performant, and reliable systems, and improving reliability in existing systems.
• Deep knowledge of design patterns and their practical application.
• Proven ability to work independently while also fostering teamwork and cross-team collaboration.
• Proven ability to mentor others and contribute to a strong team culture.
• Strong problem-solving skills, with the ability to anticipate potential issues and proactively address them.
• Excellent communication skills, capable of explaining complex technical concepts to both technical and non-technical stakeholders.