Renting to tenants is no longer “where the money is” for real estate investing. I became a multi-millionaire landlord in the 1980s by buying $10 million in rental houses. Yet, I consider this real estate investing approach the biggest mistake in my real estate investing career. I call it my “$10 Million Mistake.”Back then, real estate investing in rentals was buying a house, renting it to tenants for a rental fee that covered the mortgage, and waiting for an increase in value. Interest rates wer.