Senior Software Engineer, Backend

About Flex

Flex is a growth-stage, NYC headquartered FinTech company that is creating the best rent payment experience. It’s hard to believe that it’s 2025 and paying rent on time is expensive, inflexible, and difficult. We’re here to change that! Flex enables our users to pay rent throughout the month on a schedule that better fits their finances and budget. Our mission is to empower as many renters as possible with flexibility over their most significant recurring expense. After deliberately keeping a stealth profile as we built up unprecedented investor support and an enthusiastic user base, we are looking for motivated individuals to help us keep our mission growing. Will you be a part of the team?

About the Job

Flex is hiring Senior Software Engineer, Backend roles across four engineering teams: Consumer, Partner Integrations, Core Platform, and Risk. When you apply, you will automatically be considered for all open Senior Backend positions, and your placement will be determined based on your experience and expertise assessed during the interview process.

As a Senior Backend Engineer at Flex, you will play a key role in designing, building, and scaling backend services and APIs that power our products, partner integrations, and critical risk management platforms. You’ll collaborate closely with cross-functional teams to deliver robust, fault-tolerant systems that impact renters and partners at scale.

You will own the end-to-end lifecycle of your services- from requirements and architecture through implementation, testing, deployment, and monitoring- and help elevate the team by driving technical best practices and mentoring junior engineers.

Our Engineering Teams

Consumer Engineering

Builds the APIs and backend systems powering Flex’s main consumer-facing products. This team focuses on delivering scalable services for a seamless rent payment experience through our mobile and web apps.

Partner Integrations Engineering

Owns the backend services and integrations that connect Flex to financial partners and payment networks, ensuring reliable, secure payment flows and partner-facing experiences.

Core Platform Engineering

Maintains the backend systems and infrastructure supporting all business features, including Payments, Billing, Identity, and Partner Integrations, ensuring Flex’s foundation is robust and scalable.

Risk Platform Engineering

Develops the platforms and APIs that mitigate credit and fraud risk, enhancing customer accessibility while safeguarding against misuse through advanced rules engines, machine learning, and event data systems.

Requirements

A minimum of 4 years of experience with Java is required for this position.

What You'll Do

• Design, develop, test, and scale backend services and APIs that power Flex’s consumer products, partner integrations, risk management systems, and core platform services.
• Build and operate robust, fault-tolerant distributed systems with multiple external dependencies.
• Implement service APIs for web and mobile apps, simplifying complex rent payment processing.
• Work closely with Product, Design, Infrastructure, Sales, Partner, and Customer Success to deliver features and integrations that delight customers and partners.
• Translate feature requirements into technical specifications and lead the architecture and implementation of scalable solutions.
• Drive the full lifecycle of services and APIs you build, including deployment, monitoring, and optimization for high availability and performance.
• Contribute to evolving Flex’s service-oriented architecture, REST APIs, message queues, and scalable systems design.
• Develop risk management systems such as rules engines, ML-powered fraud detection, and high-scale event ingestion pipelines.
• Mentor junior engineers and help elevate the team’s engineering practices through code reviews, architecture discussions, and technical leadership.
• Ensure services are observable and reliable using monitoring tools and industry best practices.
• Collaborate on CI/CD improvements