Born in Philadelphia, Pennsylvania, Cassidy attended Central High School, Philadelphia. He started to rap when he was 13 years old. He rose to prominence by appearing, and winning, on The Cypher, a radio competition on 103.9 The Beat. He was also a member of the crew Larsiny with his home-town friends Cal Akbar & Shiz Lansky. Producer Swizz Beatz signed Cassidy to his Full Surface label and featured him on his 2002 album Presents G.H.E.T.T.O. Stories.

Cassidy received some fame after he won a freestyle battle with Roc-A-Fella artist Freeway. Cassidy recorded his first album, Split Personality in 2003. The record is divided into three parts: The first is credited to Cassidy and reflects his pop side, the second, credited as "Tha Problem," aims at fans of his mixtapes and the third is credited to B. Reese from fans from his early days.

Cassidy recorded the lead single "Hotel" in R. Kelly's Chicago studios with a vocal contribution by Kelly - there is also a remix featuring vocals by Trina. The song became a hit, reaching the top ten of the Billboard Hot 100 in February 2004 and was nominated for a Vibe Award in the category Coolest Collabo that year. It also reached the top 5 of the UK reggiechart and the top 40 of the Australian chart in May 2004. May 2004 was also the month Cassidy was featured on the cover of The Source magazine, alongside Lil' Flip, Young Gunz and J-Kwon.

Split Personality was released on March 16, 2004 and debuted at #2 on the Billboard 200 album charts and at #1 on the R&B/Hip Hop albums charts. The second single, "Get No Better", featured a contribution from labelmate Mashonda but did not do as well on the singles charts, reaching #82 on the Billboard Hot 100.

Cassidy released his second album, I'm a Hustla on June 28, 2005, through Swizz Beatz's label, Full Surface. It reached a chart position of #5 on Billboards Hot Rap Tracks in the United States. The album's title track features a sample from Jay-Z's "Dirt off Your Shoulder" and the song was nominated for a Vibe Award in the category Street Anthem. A remix featuring Mary J. Blige followed soon after. In 2006, the ringtone version of his track I'm a Hustla was one of the first-ever ringtones to be certified platinum.

Cassidy's third album, B.A.R.S. The Barry Adrian Reese Story was released on November 7, 2007, featuring his lead single "My Drink N' My 2 Step," which had leaked to the radio and several internet sites. He also created a B.A.R.S. Competition Freestyle Battle on October 10, 2007 live on MTV's Sucker Free. He will be featured on the AND1 Mixtape Tour 2007.