The deal will also see the family take equity ownership in the business.
The value of the deal is not being revealed.
Since its groundbreaking development of the Merry Hill shopping centre three decades ago, the Richardson family has expanded into a variety of international business interests, mostly in real estate and, more recently, in private equity, focusing on growth capital.
While remaining active developers and investors in the UK, Europe, North America, and Australasia, the Oldbury-based family business has diversified into building an international growth capital portfolio focusing on small to medium enterprises (SMEs) across a range of sectors through a direct and co-investment programme.
Despite its international reach, family still runs its business from offices on Birchley Island, in Oldbury, and from London.
Magnet Capital helps small and medium-sized companies with property development projects including new builds, conversions and refurbishments. The majority of these schemes are for housing.
Magnet Capital's CEO and co-founder, Ashley Ilsen, said: "Ultimately a short-term lender is only as good as the funding behind them so we are delighted to partner with the Richardson family.
"This puts us in a strong and highly-liquid balance sheet position, from which our developer clients will reap the benefits.
"Being a principal lender enables Magnet Capital to offer a five-star service to our borrowers and introducer partners and ensures that the projects we are funding move quickly and efficiently. The initial uptake of our development finance offering has exceeded all expectations and, despite some of the negativity in the current property market, we are all looking forward to a very successful future.”
Magnet Capital MD and co-founder Sam Howard added: “It is a tremendous privilege to be partnering with the Richardson family and to harness their deep real estate and wider business expertise.
"Over the past two years we have built a strong relationship with the Richardson team so we are excited to partner with them to launch Magnet Capital and provide SME builders and developers the finance they so desperately need."