Brennan joined Foresight in 2017 as an investment manager, before becoming senior investment manager, director and then partner in 2021.
A spokesperson for Foresight confirmed that Brennan had left the company.
Brennan was lead co-manager of the £414m Foresight UK Infrastructure Income fund, which which he launched in 2017, according to FE fundinfo.
He was also lead manager of the £77.5m Foresight Sustainable Real Estate Securities fund and manager of Foresight's inheritance tax solution, running a £350m portfolio of renewable energy and infrastructure assets.
Foresight Group eyes more M&A as acquisitions boost assets
Nick Scullion, who worked with Brennan on the income and sustainable funds, will continue to manage them alongside his other portfolios.
Brennan joined Foresight in 2017 as an investment manager, before becoming senior investment manager, director and then partner in 2021.
Prior to Foresight, Brennan was an investment analyst at SL Capital Partners, and before that he worked at the UK Green Investment Bank for three years from 2013.
Terry Smith's payout slips to £31m as Fundsmith assets and profits slide
Read more on investmentweek.co.uk