Plank is the founder and executive chairman of Under Armour, a manufacturer of sportswear, footwear and accessories, based in Baltimore, Maryland.

[16] The idea that led to Under Armour was sparked while playing for the Maryland Terrapins; Plank said he was the "sweatiest guy on the football field".

[7] A turning point for him came late in 1999, when Plank used nearly all of Under Armour's money, and employees agreed to go without pay for a few weeks, so the company could take out a $25,000 advertisement in ESPN The Magazine.

"It was important", Plank said, "that this not just be my decision.” The strategy was a success, earning the company the world's largest digital health-and-fitness community, with 150 million users.

[31] Plank and Stephanie Ruhle, a Bloomberg News journalist now with MSNBC, were questioned by lawyers in early 2023 regarding a 2017 lawsuit by shareholders of Under Armour that alleges the company artificially inflated its share price, resulting in losses for them.

According to The Wall Street Journal, court documents showed that Plank gave Ruhle a phone with a special email address to communicate with him privately and at all hours, sent her confidential financial information about the company and enlisted her help to refute concerns about slumping sales.

[37][36] On November 5, 2010, Sagamore Farms' Shared Account won the $2 million Breeders' Cup Filly & Mare Turf (GI).

[41] The building was originally built in 1914 to store port cargo and later was a community center and studio for the television series Homicide: Life on the Street; it closed in 1999.

[48] The next year, his charitable arm funded 40 summer jobs for Cherry Hill, Baltimore, public school students in the maritime transport industry.

[52] Through his Cupid Foundation, Plank donated $5 million to help create the UA House at Fayette, an East Baltimore community center run by Living Classrooms.

[53] 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.

[56] The project has converted Cole Field House, the school's former basketball arena, into the football facility, a sports medicine center and student entrepreneurship lab.

[56] In 2015, Plank donated $16 million to St. John's College High School in Washington, DC, to fund athletics, academics and entrepreneurship initiatives.

[59] On CNBC's Halftime Report in February 2017, Plank commented on Donald Trump's pro-business philosophy, saying the president was a "real asset" to the business community.

As some customers vowed to boycott the brand, three major endorsers—Stephen Curry, ballerina Misty Copeland, and actor Dwayne "The Rock" Johnson—went on Twitter to express their opposition.

In the ad, Plank said Under Armour stood for job creation, but publicly opposed the president's proposed travel ban.

He stepped down from the council following Trump's comments on violence in Charlottesville, Virginia, saying Under Armour "engages in innovation and sports, not politics".

[63] Additionally, he publicly opposed President Trump's decision to withdraw from the Paris Accord[64] and was among the Fortune 500 CEOs to sign a pledge to promote workplace diversity and inclusion.