Alongside being a partner, Rennie is the Head of Philanthropy at C. Hoare & Co., a role he has held since 2018. He also serves as the Chair of Hoares Trustees Limited, which oversees the bank’s donor-advised fund, the Master Charitable Trust, and the Chair of Philanthropy Impact, further establishing his leadership in the field.

His expertise in philanthropy has earned him multiple recognitions, including being named Philanthropy Adviser of the Year at the Spear's Wealth Management Awards in 2019 and the CityWealth Magic Circle Awards in 2023. Since 2019, he has been consistently listed as a Top Recommended Philanthropy Adviser in the Spear's 500 guide, and he was recognized in the PAM Top 40 Under 40 in 2021.

Under his leadership, C. Hoare & Co. received the Best Philanthropy Engagement award at the Managing Partners' Forum Awards and was named Outstanding Private Bank for Philanthropy at the London Private Banking & Wealth Management Awards in 2019.

Rennie is a frequent speaker at prominent industry events, including the Association of Charitable Foundations (ACF), Camden Wealth, Family Business UK (FBUK), Global Family Office Community (GFOC), London Business School (LBS), and Sustainable Finance Week. He has also contributed to research publications by CAMRADARA, Cazenove, the International Institute for Management Development (IMD), and KPMG.

His commentary on philanthropy and impact investing has been featured in national newspapers and trade publications such as the Financial Times, Telegraph, and WealthNet.