About Us

Onside is a high-touch finance partner purpose-built for tech startups.

Running a VC-backed tech company demands speed, clarity, and confidence in decision making. Your finance function shouldn’t slow you down, it should support better decisions while you focus on growing your business.

Onside blends exceptional client service with accounting, tax advisory and CFO expertise, supporting founders from pre-seed to Series C. We understand the pace and pressure of scaling and operate alongside founders as an embedded finance partner.

Onside combines modern systems with human oversight. Our team operate at the pace and the standard that tech startups need to scale. Our ways of working are designed to streamline your finance operations, reduce friction and improve visibility.

Onside recently entered a strategic partnership with Multiplier, a San Francisco–based technology company backed by Ribbit Capital and Lightspeed Venture Partners, to accelerate the development of technology that improves our client and staff experience.

Onside Accounting was founded in 2021 by Martin Brennan FCCA, bringing experience from operational leadership and technical roles within high-growth accounting practices. That background shaped how the firm was built, with a strong focus on quality, scalable processes, and developing people as the foundation of long-term client service.

Today, Onside is deeply embedded in the tech and venture ecosystem. Martin is an active angel investor in high-growth tech and fintech businesses and works closely with VC, angel, and founder networks across the UK and internationally. He also sits on the Advisory Board of Apron, a Series B fintech, and is a member of ACCA’s Global Council.

This proximity to founders, operators, and investors means we understand how investors think, how scaling companies are evaluated, and what “good” looks like at each stage of growth. That insight directly informs how we support our clients, from early-stage structuring through to later-stage reporting, governance, and strategic finance.

Developing future accountants and tax advisors remains a core part of our culture, reflected in the depth, quality, and continuity of the teams we build around our clients.

From incorporation to expansion, we deliver expert finance and tax support that scales with you – Giving founders clarity, confidence and control over their growth.