Principal Data Scientist in London

About Marshmallow
We exist to make migration easy. A systemic problem of this magnitude requires a team of curious thinkers who relentlessly pursue solutions. Those who constantly challenge the why, dismantle assumptions, and always take action to build a better way. A Marshmallow career is built on a cycle of continuous growth, with learning at its core. You will be challenged to raise the bar on your capabilities and supported with the right tools and guidance to do so. This ensures you can deliver impactful work and drive change.

Data Science at Marshmallow
Our Data Science team partners across the business to turn data into better decisions, smarter products, and simpler customer journeys. We work closely with Product, Engineering, and Operations to build and ship models and AI systems that are reliable in production and deliver measurable impact. Within Data Science, this role sits in Claims, supporting Claims Fraud and the broader ambition to automate more of the claims journey. Claims is one of Marshmallow's most important customer touchpoints, and we’re looking for a Principal Data Scientist who can provide technical leadership across traditional ML and Generative AI, bring system-level thinking to how we scale decisioning, and confidently challenge proposals to ensure we build robust, sustainable solutions.

What you’ll be doing

• Provide technical leadership for data science across Claims Fraud, shaping the approach to risk decisioning and fraud detection in partnership with Product and Engineering.
• Design, build and iterate on production ML and Generative AI/LLM systems that support claims validation and automation.
• Collaborate closely with other Claims data scientists to bring system-level thinking to how models, data and workflows fit together, identifying architectural improvements needed to scale decisioning and reduce time-to-production.
• Be vocal about the platform and tooling investments needed (monitoring, feedback loops, QA) to achieve AI-driven end to end claims automation.
• Advocate for robust, scalable, and strategically aligned technical solutions in cross-functional discussions, ensuring current systems and infrastructure contribute to the multi-year vision for automated claims handling.
• Set a high bar for statistical rigour, experimentation and measurement, helping improve how Claims performance and uncertainty are understood and communicated to senior stakeholders.

Who You Are

• You think in systems: you can connect the dots between data science, engineering, and product to shape scalable solutions that build on each other over time.
• You’re confident in challenging assumptions and pushing for the right approach, using strong communication skills to influence stakeholders across seniority levels and disciplines with clear, pragmatic reasoning.
• You thrive in ambiguity and change, staying resilient and effective during transitions while bringing structure, clarity, and momentum to complex problem spaces.
• You’re motivated by real-world impact, partnering closely with cross-functional teams to drive meaningful automation and better customer outcomes across the claims journey.

What You’ll Bring

• Significant commercial experience delivering end-to-end Machine Learning solutions, from problem framing and experimentation through to production deployment and ongoing monitoring.
• Hands-on experience building and shipping Generative AI systems in production (not just prototypes), including evaluation, safety/quality considerations, and integration into customer or operational workflows.
• Strong statistical and modelling foundation, with experience in risk-based decisioning under uncertainty (e.g., fraud, credit, insurance, or other regulated domains).
• Proven ability to influence technical direction across Data Science and Engineering, including shaping scalable model/service integration patterns and challenging proposals to drive robust, long-term solutions.
• Strong stakeholder management skills, with confidence communicating trade-offs and pushing back constructively with Product and Engineering to ensure high-quality outcomes.

Perks of the job

• Flexible working: Spend three days a week with your team in our collaborative London office, and own your own working hours.
• Competitive bonus scheme - designed to reward and recognise high performance.
• Flexible benefits budget - £50 per month to spend on a Ben Mastercard meaning you get your own benefits budget to spend on things you want.
• Mental wellbeing support – Access therapy and mental health sessions through Oliva.
• Learning and development – Personal budgets for books and training courses to help you grow in your role.
• Private health care - Enjoy all the benefits Vitality has to offer.
• Medical cash plan - To help you with the costs of dental, optical and physio.
• Tech scheme - Get the latest tech for less.
• Plus all the rest; 33 days holiday (including bank holidays), pension, cycle to work scheme, monthly team socials and company-wide socials every month.

We are able to offer sponsorship and/or a visa for this position.

Our process

• Initial call with a member from our Talent Team (30 mins).
• Past Experience interview with Hiring Manager (60 mins).
• Systems Design interview with a couple of the team (60 mins).
• Technical interview with a couple of the team (60 mins).
• Culture interview (60 mins).
• Background checks.

As part of our commitment to maintaining a safe and trustworthy environment, we’ll carry out standard background checks, including a DBS and a Cifas check.

Everyone belongs at Marshmallow
At Marshmallow, we want to hire people from all walks of life with the passion and skills needed to help us achieve our company mission. To do that, we’re committed to hiring without judgement, prejudice or bias. We encourage everyone to apply for our open roles. Gender identity, race, ethnicity, sexual orientation, age or background does not affect how we process job applications. We’re working hard to build an inclusive culture that empowers our people to do their best work, have fun and feel that they belong.

Recruitment privacy policy
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