Feld began financing technology startups in the early 1990s, first as an angel and later an institutional investor. Feld was an early investor in Harmonix, Zynga, MakerBot, and Fitbit. In 1987, Feld co-founded Feld Technologies — a software application development firm — while he was a student at Massachusetts Institute of Technology. In 1993, the company was acquired by AmeriData, where Feld took the role of chief technology officer
In the late 1990s and early 2000s, Feld helped operate Interliant, an application service and hosting provider.
Interliant failed in 2002, following the collapse of the dot-com bubble. Feld openly talks about his experience at Interliant, using it as an example of entrepreneurial failure.
Before co-founding Foundry Group in 2007, Feld co-founded Venture capital firms Intensity Ventures and Mobius Venture Capital. After its inaugural $225 million fund, Foundry Group has raised three more funds of $225 million each — in 2010, 2012, and 2013.
In 2006, Feld helped David Cohen co-found Techstars, an early-stage venture fund and startup accelerator.
Feld serves on the boards of nonprofits National Center for Women & Information Technology, Startup Colorado, and Uttar Pradesh Global. On October 7, 2015, at a talk Brad give in which he participated in a Q&A session, he was asked "What do you want your legacy to be?", to which he answered: "I don"t care what my legacy is". In the comment section of his blog the next day, he added "I care about what I do.
I don"t care about the notion that it creates a legacy.".