Underwood was born in Philadelphia on March 29, 1977, the youngest child of Paul Underwood, a former insurance salesman, and Eileen Underwood, an information specialist at IBM. The family later moved to upstate New York, and later to his father's hometown of Fayetteville, North Carolina, after Eileen Underwood was laid off from her job.
Underwood attended E.E. Smith High School. He played college football at Michigan State University under head coach Nick Saban. As a junior starter he registered 57 tackles (12 for loss) and 8 sacks. He missed his senior season because of an ankle injury, although it has been speculated that it was not the real reason.
A few months after Underwood was accepted into Michigan State, his father died from leukemia at the age of 47.
Underwood was selected in the first round (29th overall) of the 1999 NFL Draft after his stock rose with his pre-draft workouts. He signed a five-year, $5.3 million contract with a $1.7 million bonus on August 1, 1999, and walked out of training camp the next day after the first practice and never returned. He forfeited a $1.75 million bonus saying he could not resolve the conflict between playing football and serving his Christian faith. Although he eventually returned, he was released later that month and had to return his signing bonus to the Vikings.
He was claimed off waivers by the Miami Dolphins as 23 teams passed on him. He was signed for $395,000, played in only one preseason game before injuring his shoulder and also showed a lack of focus on football. Multiple times during team meetings, Underwood was found not taking notes, but instead writing about the apocalypse. He only played one preseason game for the Dolphins before getting injured.
In September 1999, Underwood was arrested by police for failure to pay child support for his 17-month-old twins.
Underwood later spent two months in protective care and was diagnosed with bipolar disorder. After he escaped from a psychiatric care facility, he was released by the Dolphins on December 17, 1999. On March 10, 2000, he signed a two-year contract with the Dallas Cowboys, who took a chance on him because of the earlier success the team had with Alonzo Spellman who also suffered from bipolar disorder. During the 2000–2001 seasons, he was moved from defensive end to defensive tackle and had 21 tackles and four sacks in 19 games as a reserve player. In January 2001, he tried to kill himself for the second time by running into traffic twice on a busy suburban highway. The Cowboys released him later that month after he missed practices.
He served stints in the Dallas County Jail for aggravated robbery, assault on public servant, and evading arrest starting in 2002. Underwood spent his time in a closed custody cell.
After four years out of football, he resurfaced in the CFL with the Ottawa Renegades on May 24, 2005. However, he was cut during the preseason on June 13.
In 2007, Underwood entered the Philadelphia Industrial Correctional Center, and left in February 2011.
In 2007, Charles Robinson of Yahoo! Sports listed Underwood as one of the worst first-round picks (at the 29th slot) since the AFL-NFL merger. According to the article, Underwood missed most of his senior year, and his coaches at Michigan State warned NFL scouts that he was not mentally stable enough to play in the NFL. In Robinson's view, by ignoring these warnings the Vikings made "arguably the dumbest pick ever made in the first round." In a September 28, 2010 article Yahoo Sports noted Underwood's mental health issues as a case in point in regard to the NFL falling short in helping players who suffer from various mental illnesses that are in no small part connected to the pressures of the job of an NFL athlete. The 2010 article also pointed out that "Underwood was tormented by visions of the apocalypse. He used to write notes discussing the end of the world on pieces of paper the size of postage stamps. When he was in a normal state, he could be engaging in conversation. He was funny and intelligent. He also exuded physical confidence and had extraordinary talent."