Selected 37th overall in the 1983 NFL draft, he spent the first ten seasons of his career with the New York Giants where he accumulated 79.5 sacks, ranking him third in team history.

He also has spent time as a football camp instructor, a football head coach, written a couple of books,an Executive in Residence at Seton Hall University, a adjunct professor at Seton Hall University , and a radio broadcaster who made several appearances on The Howard Stern Show for 6 years starting in 1985-1991.In 2013, Marshall was diagnosed with chronic traumatic encephalopathy (CTE), a degenerative brain disease detected in a growing number of retired NFL players.

mMarshall is currently on the board of Carver Bank of Harlem, PikeTx.com, The Concussion Legacy Foundation, CaringKind of New York, and several other non profits.

He is also an Outside Advisor at Rockefeller Capital Management of New York City where he works alongside Edward Moldaver, James Lee and Steve Cohen.

[9] After trimming down, Marshall made an impact and played strongly against the run[10] but was unable to mount much of a pass rush and finished with only 0.5 sacks on the season.

He also made several key tackles to help the Giants preserve a 16–13 win over the Los Angeles Rams during the wild-card round of the playoffs.

[11] Marshall's emergence as a pass rusher continued in 1985 as he racked up 15.5 sacks, earning him a trip to the Pro Bowl and recognition as NFL defensive lineman of the year.

[2] Marshall repeated both honors in 1986,[12] teaming up with Hall of Fame linebacker Lawrence Taylor to form a potent pass rush on the right side of the Giants defense.

The Giants went 13–3 during the regular season and advanced to the NFC Championship Game where the two-time defending Super Bowl champion San Francisco 49ers awaited them.

[31] In 2015, he was named Director of Strategic Initiatives and Brand Ambassador for the publicly traded restaurant chain The Original Soup Man.

[35] In 2013 Marshall was diagnosed with chronic traumatic encephalopathy (CTE), a degenerative brain disease linked to repeated head trauma that has afflicted many former football players.

Marshall says he began to notice symptoms such as memory loss, severe headaches, and mood swings in 2006, and that his erratic behavior took a major toll on the personal relationships in his life, including his marriage.

[36][37][38] Marshall was part of a class action lawsuit against the NFL that initially resulted in a $765 million settlement, but was later amended to allow for a payout expected to exceed $1 billion.

[36][39] Marshall is involved with efforts to educate the public about concussion and head trauma issues, serving as a speaker for both the Practice Like Pros[40] and Brain Unity Trust[41] organizations.

I would believe these men have a lot of respect for what I did as a player, both on and off the field, within the community, and the surrounding area, and what I continue to do to support the New York football Giants.

[44][45] Marshall has spoken publicly in support of cannabis as an effective treatment option for mitigating the symptoms of head trauma-related health issues.

[44] Marshall served as a keynote speaker and moderator at the Cannabis World Congress and Business Expo in New York City on June 15–17, 2016.

[49] During his time as a player, Marshall was a frequent presence on The Howard Stern Show and he co-hosted NY Football with Dick Lynch.