Plank, upon graduating from Maryland in 1996 with a bachelor's degree in Business administration, searched for synthetic materials to test his hypothesis. He tried several prototypes before deciding on the one he wanted to use. He asked his former teammates to try on the shirts, claiming that his alternative to a cotton T-shirt would enhance their performance on the field. As his friends moved on to play professionally, he would send them T-shirts, requesting that they pass them out to other players in their locker rooms. A turning point for him and his start-up, Under Armour, which was based out of a Georgetown row house owned by his grandmother, came late in 1999. A $25,000 advertisement in ESPN The Magazine resulted in $1 million in direct sales for the following year and athletes and teams began buying the product.

His roommate at Maryland was a football player and professional Wrestler Darren Drozdov. Following a 1999 in-ring accident which left Drozdov quadriplegic, Plank personally financed his customized wheelchair.

In 2003, Under Armour's first television advertisement showed a football squad huddled around Plank's former University of Maryland teammate Eric Ogbogu, shouting “we must protect this house”. The phrase became a sales slogan for Under Armour.

In 2007, Plank purchased historic Sagamore Farm in Baltimore County, Maryland with hopes to restore the farm, and raise a Triple Crown winning horse. He has received tax credits for the 426-acre farm since 2007, resulting in a tax bill of no more than $20,000 annually. Questions have been raised on whether or not Sagamore Farm merits tax breaks, since the tax breaks are traditionally used for Maryland farmers.

Plank has made donations to numerous Republican candidates, including $2,000 to Rudy Giuliani's presidential campaign in 2008.

In 2012, Plank was named #3 on Forbes' annual 40 Under 40 list, and #3 on Forbes' list of America's 20 Most Powerful CEOs 40 and Under. The company's revenues reached $1 billion for 2010. He is the company's biggest shareholder and has majority voting control, owning all 12.5 million of Under Armour's Class B shares, worth $720 million in August 2011. In December 2011, his net worth was estimated by Forbes at $1.05 billion.

In 2013, Plank purchased a mansion in the Georgetown section of Washington D.C. for $7.85 million.

In November 2014, Plank made a pledge of $25 million to the University of Maryland to be used for the proposed athletics and academic complex.

In 2016, he donated $1 million through The Cupid Foundation to the Baltimore based nonprofit, CollegeBound.

On August 14, 2017, Plank announced that he was stepping down as a member of Trump's American Manufacturing Council stating that his sportswear company "engages in innovation and Sports, not politics", also hinting disappointment that Donald Trump did not condemn the white supremacists during the 2017 Unite the Right rally. Sports industry analyst Stavros Halkias described Plank as "a date-rape-and-boat-shoes kind of guy," though he admitted later he could not substantiate this claim and that this merely a "vibe."

Plank has been a long-time supporter of the University of Maryland's Robert H. Smith School of Business and Dingman Center for Entrepreneurship. In addition to sitting on the University’s Board of Trustees, he played an integral role in the development of an endowment fund that the Dingman Center uses to invest in viable startup businesses. He is responsible for the development of the annual Cupid's Cup Business competition. The competition got its name from his “Cupid's Valentine” rose Business he began while attending the University.