Fields grew up in Richmond Hill, Queens, New York. After graduating from the University of Pennsylvania in 1959 he attended Harvard Law School, but left during his first year. He moved to Manhattan's Greenwich Village in 1960, briefly enrolled at New York University, and became involved with the burgeoning downtown arts and music scene. He eventually became a pivotal member of Andy Warhol's Factory social circle. He hosted a radio show on New Jersey's WFMU during its groundbreaking 1968-1969 free-form years.

Fields shared a loft with Warhol actress Edie Sedgwick, and wrote an account of the Warhol-sponsored Velvet Underground during their early years. (He later penned the liner notes for the band's historic Live At Max's Kansas City album, recorded in 1970 but released in 1972 after the band broke up.) Fields was one of the first people in the music business to be openly gay, at a time when most were closeted.

He was hired by Elektra Records as a publicist. Elektra, which had primarily been a folk music label, was having huge success in the rock record market with The Doors and hired Fields to publicize the band, despite the fact (discussed by Fields in numerous interviews) that he and lead singer Jim Morrison did not like each other. In September 1968 Fields visited Detroit and Ann Arbor on the recommendation of two fellow DJs at WFMU. He recommended to Elektra that the label sign the MC5 and The Stooges. Both bands served as major inspirations for the US and UK punk music movements of the mid- and late-1970s.

IN 1975 Fields discovered the Ramones at the club CBGB, and helped get them signed to Sire Records. As the band's co-manager with Linda Stein, Fields in 1976 took the band to England, where they had an enormous impact, inspiring the nascent UK punk movement, including such performers as the Sex Pistols and The Clash. Under Fields' management the Ramones recorded Ramones, Leave Home, and Rocket to Russia. The 1980 Ramones album End of the Century includes the track "Danny Says", with Fields being the "Danny" referred to in the title.

Fields later managed Steve Forbert and The Modern Lovers. In 1990, Fields discovered singer-songwriter Paleface at a New York open mic. He became Paleface's manager and helped the young artist get signed to Polydor Records and later on to Sire Records (formerly Elektra Records).

After leaving the music business, Fields co-authored Dream On, the biography of Warhol film personality Cyrinda Foxe, the wife of Aerosmith's Steven Tyler. He subsequently wrote Linda McCartney: A Portrait, which was turned into a television miniseries by CBS. Fields currently lives in New York City.

Interviews with Fields are included in the documentaries: Icon (1995), We're Outta Here! (1997), 25 Years of Punk (2001), A True Testimonial (2002), The Story of the Ramones (2003), and Danny Williams and the Warhol Factory (2007), It's Alive 1974-1996 (2007), and Lords of the Revolution: Andy Warhol (2009).