Senior Software Engineer (C#)

Job Description

NMI is a leading provider of payment gateway solutions, specialising in card-present solutions and payment processing. Our innovative technologies empower businesses to securely accept payments and optimise transaction processing. With a focus on reliability, security, and innovation, we continuously push the boundaries of what’s possible in the payments industry.

We are seeking an experienced Senior Software Engineer to join our Core Gateway team. The Core Gateway team is responsible for the development, maintenance, and evolution of NMI’s primary payment gateway platforms, powering high-volume, business-critical transaction processing for merchants globally.

This team plays a pivotal role in ensuring our gateways operate reliably, securely, and at scale. By continuously improving performance, resilience, and functionality, the Core Gateway team enables seamless payment operations and supports our customers in an increasingly digital economy.

The ideal candidate:

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# (preferred) or PHP, 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

As well as being a part of something exciting everyday, you will also receive the following benefits:

• Annual bonus scheme dependent on individual and company performance
• Annual salary of £50,000 - £65,000
• 25 days holiday each year (+ bank holidays + 1 day after each year of service with up to a max. of 30 days)
• Workplace pension scheme
• Private medical insurance (upon 30 days of employment)
• 7 hours per day, 35 hours per week
• A remote first culture
• Great work-life balance with our Flexi-time policy
• Family Friendly policies (Enhanced Maternity and Paternity Pay and Shared Parental Leave).
• A chance to develop with an allocated company training budget
• Bike2Work Scheme
• Lifeworks, an Employee Assistance Programme which offers wellbeing, family and financial support services, such as assessments, resources and even 1:1 counselling sessions. It also offers interesting perks such as discounts on gyms, restaurants, high street retailers and cinema tickets
• A strong commitment to employee wellbeing including mental health first aiders
• Employee referral scheme with generous financial reward
• Bonusly colleague reward scheme

We’re looking for creative and passionate people who share our vision of making payments easy. If that sounds like you and you meet the requirements above, then please click on 'Apply for this job'!

We are an Equal Opportunities employer and will provide reasonable support throughout the recruitment process to applicants who have a disability. Please let us know in advance so that any support, aids or adaptations can be put in place to assist you.

Please be aware that all offers of employment are made subject to receipt of satisfactory background and financial checks.

About us

NMI enables our partners with choice, and challenges the one-size-fits-all approach to payments. You've probably used NMI in the last 24 hours without even realising it. We’re the platform that powers success for innovative tech created by SMBs, entrepreneurs and fintech start-ups. We’re creative problem solvers who help visionaries smash through boundaries and think beyond what’s possible so they can think about what’s next. But we’re not just built for the tech savvy. We democratise the latest payments technology so that everyone can realise the benefits of easy payments across the full spectrum of commerce. We’re all about enabling more payments in more ways and more places.

Please note that in compliance with the data protection regulations within your jurisdiction, any personal information submitted with your job application may be collected and used by NMI for the purpose of recruitment and employment-related activities. By submitting your application, you acknowledge and provide explicit consent to the processing of your personal information as described in our privacy policy found on our website. For more information on how we process your information, please read our privacy policy here: https://www.nmi.com/legal/privacy-policy/