About Zeffy

Zeffy is the fastest-growing nonprofit platform in North America, helping organizations raise funds, run events, and engage donors. We are already working with more than 80,000 nonprofits and are doubling annually.

Millions of people dedicate their lives to causes beyond profit, running schools, shelters, and community initiatives. Yet most still lack modern tools. We believe the greatest opportunity for technology today is to amplify the impact of nonprofits.

Built on a sustainable, self-funded model, Zeffy reaches over 1M donors monthly and grows profitably, giving us the freedom to focus fully on our mission.

Nonprofit workers are heroes. They deserve the best technology, and we’re hiring top minds and top hearts to build it for them.

About the role

As a Senior Fullstack Data, you’ll bring seniority and end-to-end fullstack data expertise to a data team on a mission to turn data into impact.

Over the past few months, our team has:

• Built a fraud detection model that flags suspicious transactions in real time and reduces manual review.

• Developed a non profit segmentation model to help marketing personalize campaigns and improve conversion rates.

• Experimented with a recommendation system to optimize donation tip suggestions and boost nonprofit revenue.

• Improved data reliability and monitoring across our stack with automated anomaly detection and alerts.

If you join us, you’ll take ownership of various projects that directly impact our growth, from fraud prevention to smarter donation experiences, moving quickly from idea to production. The role is very hands on and full stack, in a sense that you will :

• Build and maintain all the parts of our data stack (ingestion, modeling, data viz, ML, …)

• Discuss with our business stakeholders to translate business needs into data requirements

• Build and deploy data solutions to answer those needs

• Communicate your results and follow the business impact

• Mentor and help the whole data team grow

Data team principles at Zeffy

• Impact first: We optimize for maximum business impact with minimal effort, what we call Zeffy perfectionism. We have no problem shifting the priorities if we realize that the impact is higher or lower than expected. We are a small team of 2 and we need to be cautious about how we use our time to maximize our impact on the business

• Trust and ownership: Every project has a clear owner with full autonomy to deliver results : collaboration is encouraged, but accountability is clear. You are responsible of the project for the whole development cycle, from gathering the needs to designing the solution to implementing the solution to monitoring the impact

• Decentralized organization : Everyone at Zeffy is autonomous at monitoring the performance of their team with data and answer their basic questions. The role of the data team is to build the foundations for the teams to increase this level of autonomy. It comes with a clear and documented data model and scoring and classification projects to bring interpretability in our data

Our data Stack

• Languages & Frameworks: Python, SQL, scikit-learn, XGBoost, PyTorch / TensorFlow

• Data & MLOps: Snowflake, dbt, AWS, Metabase, Metaplane, Fivetran

Preferred experience (qualifications)

We’re looking for someone with 4+ years of experience in ML engineering, data science, or applied data roles, ideally in a SaaS, fintech, or high-volume B2C/B2B tech environment.

You’ll fit right in if:

• You’ve deployed ML models into production and monitored their performance.

• You understand how to connect ML outputs to business decisions.

• You’re comfortable moving between data pipelines, APIs, and model code.

• You bring a software engineering mindset to ML (CI/CD, testing, version control).

• You’ve mentored or guided teammates on data and ML best practices.

• You can clearly communicate complex ML results to non-technical audiences and make insights actionable.

💡 Research shows that candidates from underrepresented backgrounds often don't apply for roles if they don't check every box. If this applies to you and you are interested in the position, we'd love to hear from you!

What’s in it for you

• 🏖️ 5 weeks holiday + RTT per calendar year

• 💰 Competitive stock option program

• 🏥 Health insurance (with Alan)

• 🚲 Allowance for ecological transport #bike #transit

• 🤝 Team volunteering four times per year

• 🏢 Offices located in the heart of Paris 11ème, with the possibility of flexible, hybrid work

• 🌍 Offsite summit once a year with the entire team

• ✨ Work with individuals driven to make a profound and positive impact on the world!

Recruitment Process

• Call with Recruiter (45 min)

• Meeting with Hiring Manager (1.5h)

• Case Study (2h)

• Cultural interview with leadership (1h)

• Team lunch & Reference check

• Job offer