Mos Def attended middle school at Philippa Schuyler Middle School 383 in Bushwick, Brooklyn where he picked up his love for acting. After returning from filming You Take the Kids in Los Angeles, and getting into a relationship with an older girl, Mos Def dropped out of high school during sophomore year. Growing up in New York City during the crack epidemic of the 1980s and early 1990s, he has spoken about witnessing widespread instances of gang violence, theft and poverty in society, which he largely avoided by working on plays, Off-Off-Broadway and arts programs. In a particularly traumatic childhood experience, Mos Def witnessed his then five-year-old younger brother Ilias Bey (b. Denard Smith) get hit by a car. Bey, who later adopted the alias DCQ, was described by Smith as "my first partner in Hip Hop".

Prior to his career in music, Mos Def entered public life as a child actor, having played roles in television movies, sitcoms and theater, some of which were under the name Dante Beze. At the age of 14, he appeared in the TV movie God Bless the Child, starring Mare Winningham, which aired on ABC in 1988. He played the oldest child in the 1990 family sitcom You Take the Kids shortly before it was cancelled. In 1995 he played the character Dante, Bill Cosby's sidekick on the short-lived detective show, The Cosby Mysteries. In 1996 he also starred in a Visa check card commercial featuring Deion Sanders. In 1997 he had a small role alongside Michael Jackson in his short film and music video Ghosts (1997).

About.com ranked him #14 on their list of the Top 50 MCs of Our Time, while The Source ranked him #23 on their list of the Top 50 Lyricists of All Time. AllMusic called him one of the most promising Rappers to emerge in the late 1990s, as well as one of hip-hop's brightest hopes entering the 21st century. Mos Def has influenced numerous hip hop artists throughout his career, including Lupe Fiasco, Jay Electronica, Kid Cudi, and Saigon. Kendrick Lamar has also mentioned Mos Def as a very early inspiration and someone he listened to "coming up" as a young Rapper, though he denied being a part of the conscious rap movement.

Mos Def began his rap music career in 1994, forming the rap group UTD (or Urban Thermo Dynamics) along with younger brother DCQ and younger sister Ces. In 2004, they released the album Manifest Destiny, their first and only release to date. The album features a compilation of previously unreleased and re-released tracks recorded during the original UTD run.

Mos Def married Maria Yepes in 1996, and has two daughters with her: Jauhara Smith and Chandani Smith. He filed for divorce from Yepes in 2006. The former couple made headlines when Yepes took Mos Def to court over failure in child-support obligations, paying $2,000 short of the monthly $10,000 he is ordered to pay. Mos Def has four other children.

Mos Def signed with Rawkus Records and formed the rap group Black Star with Talib Kweli. The duo released an album, Mos Def & Talib Kweli Are Black Star, in 1998. Mostly produced by Hi-Tek, the album featured the singles "Respiration" and "Definition", which both reached in the Billboard Hot R&B/Hip-Hop Songs chart.

Mos Def released his solo debut album Black on Both Sides in October 1999, also through Rawkus. The single "Ms. Fat Booty" charted, while the album reached #25 on the Billboard 200. Around this time he also contributed to the Scritti Politti album Anomie & Bonhomie.

Mos Def is well known for his left-wing political views. In 2000, Mos Def performed a benefit concert for death row inmate Mumia Abu-Jamal.

In 2001, he took a supporting role to Beyoncé Knowles and Mehki Phifer in the MTV movie Carmen: A Hip Hopera as Lt. Miller, a crooked cop.

In 2002, he played the role of Booth in Suzan-Lori Parks' Topdog/Underdog, a Tony-nominated and Pulitzer-winning Broadway play. He and co-star Jeffrey Wright won a Special Award from the Outer Critics Circle Award for their joint performance. He played Left Ear in the 2003 film The Italian Job. That same year he appeared in the music video You Don't Know My Name of the song by Alicia Keys.

In September 2005, Mos Def released the single "Katrina Clap", renamed "Dollar Day" for True Magic, (utilizing the instrumental for New Orleans Rappers UTP's "Nolia Clap"). The song is a criticism of the Bush administration's response to Hurricane Katrina. On the night of the MTV Video Music Awards, Mos Def pulled up in front of Radio City Music Hall on a flatbed truck and began performing the "Katrina Clap" single in front of a crowd that quickly gathered around him. He was subsequently arrested despite having a public performance permit in his possession.

In October 2006, Mos Def appeared on 4Real, a documentary television series. Appearing in the episode "City of God", he and the 4Real crew traveled to City of God, a favela in Rio de Janeiro, Brazil, to meet Brazilian MC MV Bill and learn about the crime and social problems of the community.

On September 7, 2007, Mos Def appeared on Real Time with Bill Maher where he spoke about racism against African Americans, citing the government response to Hurricane Katrina, the Jena Six, and the murder conviction of Mumia Abu-Jamal. He appeared on Real Time again on March 27, 2009, and spoke about the risk of nuclear weapons.

In 2008, Mos Def starred in the Michel Gondry movie Be Kind Rewind, playing a video rental store employee whose best friend is played by co-star Jack Black. He also portrayed Chuck Berry in the film Cadillac Records, for which he was nominated for a Black Reel Award and an Image Award.

In 2009, he appeared in the House episode entitled "Locked In" as a patient suffering from locked-in syndrome. His performance was well-received, with E! saying that Mos Def "delivers an Emmy-worthy performance." He was also in the 2009 film Next Day Air.

In 2010, he appeared on the children's show Yo Gabba Gabba! as Super Mr. Superhero. He also appeared in A Free Man of Color, John Guare's play at the Vivian Beaumont Theatre.

In September 2011, Mos Def joined the cast of the environmental children's hip hop musical - Pacha's Pajamas: A Story Written By Nature - as narrator. He stated "the earth was given as a trust to mankind, so we have a responsibility to look after it, take care of it, treat it with respect 'cause it's a gift from the creator to us... We're so dependent on the natural world. The natural world's also dependent on us... If we don't treat it good, it's not gonna treat us very good either."

In January 2012, it was reported that Yasiin Bey and Talib Kweli had begun "to resurrect" Black Star.

In July 2013, Mos Def, under the new name Yasiin Bey, appeared in a short film released by the human rights organization Reprieve, depicting the forced-feeding methods used at the Guantanamo Bay detention camps. This transpired after a document containing the military instructions for the procedure was leaked.

In 2015, Yasiin Bey was featured on A$AP Rocky's second studio album At. Long. Last. ASAP, on the track "Back Home", alongside Acyde and the deceased A$AP Yams. Bey again revived his Mos Def moniker for two new songs in August 2015, titled "Basquiat Ghostwriter" and "Sensei on the Block", respectively.

In January 2016, Mos Def was ordered to leave South Africa and not return for five years, having stayed in the country illegally on an expired tourist visa granted in May 2013. Also that month, he was charged with using an unrecognized World Passport and having lived illegally in South Africa since 2014. Mos Def had reportedly recruited Kanye West to help defend him, and posted a message on West's website announcing his retirement from show Business. There is an ongoing court case in relation to immigration offences involving the Artist and his family.

In February 2018, Yasiin Bey announced, on stage, a new Black Star studio album to be released in 2018. The album will be produced by Madlib. No specific release date was made available.