Reynolds' first experience writing about music was with Monitor, a fanzine he helped to found in 1984 while he was studying history at Oxford. The publication only lasted for six issues. When it was discontinued in 1986, Reynolds was already making his name writing for Melody Maker, one of the three major British music magazines of the time (the other two being the New Musical Express and Sounds). His early Melody Maker writings often contained strong criticisms of the concept of "soul" (then being heavily promoted by the NME), and of the somewhat earnest politicisation associated with the Red Wedge movement. He has since stated that his apparent de-politicisation at the time was mainly a result of his sheer despair at Thatcherism and desire to escape - into a parallel world which was, as in the title of his first book, "blissed out". He also wrote a number of articles analysing what has since become known as twee pop from a somewhat sociological perspective, seeing in it a desire to escape the dominant 1980s values of commercialism and Americanisation and to return to a perceived innocent past.

In 1990, Reynolds left Melody Maker (although he would continue to contribute to the magazine until 1996) and went freelance, splitting his time between London and New York. The same year, he published Blissed Out: Raptures of Rock, a collection of his writings from the 1980s. Until his switch to freelance writing, Reynolds had focused mainly on rock, punk rock, post-punk, and pop. But in the early 1990s, he became involved in rave culture and the electronic dance music scene. He began writing about electronic music and became one of the foremost music critics of electronic dance music.

In 1994, Reynolds moved permanently to the East Village in Manhattan. In 1995, he co-authored The Sex Revolts: Gender, Rebellion and Rock'N'Roll with his wife, Joy Press. Sex Revolts is one of the major reasons why Reynolds has gained a reputation for the discussion of gender roles in music; the book is a critical/clinical analysis of the theme of gender in rock.

In 1998, Reynolds published Energy Flash: a Journey Through Rave Music and Dance Culture in the UK, and became a senior editor at Spin magazine in the US. In 1999, he went back to freelance work and published the American version of Energy Flash in abridged form, titled: Generation Ecstasy: Into the World of Techno and Rave Culture. Energy Flash is a comprehensive history of what became rave music, starting with Detroit techno and Chicago house and tracing the evolution of the music back and forth across the Atlantic, all the way up to the late 1990s. Reynolds combines analysis of the music, social background and history, and interviews with big names of the day. One of the most notable aspects of the book is Reynolds' analysis of the role of drugs, particularly ecstasy, in rave culture.

In 2005, the UK version of Rip It Up and Start Again: Post Punk 1978-1984 was published; the American version came out in early 2006. Rip it Up is a history of post-punk, defining the genre and placing it in the context of 1970s and 1980s music.

Reynolds has continued writing for prominent magazines, as well as his blog, Blissblog.

In 2007, Reynolds published Bring the Noise: 20 Years of Writing about Hip Rock and Hip Hop in the UK, a collection of his writing themed around the relationship between white bohemian rock and black street music. In 2008, an updated edition of Energy Flash was published, with new chapters on the ten years of dance music following the appearance of the first edition.

He contributed a chapter to Sound Unbound: Sampling Digital Music and Culture (The MIT Press, 2008), edited by Paul D. Miller a.k.a. DJ Spooky.

Reynolds has become well-known for his incorporation of critical theory in his analysis of music. He has written extensively on gender, class, race, and sexuality, and their influence on music. The Sex Revolts discusses gender in rock music. In his study of the relationship between class and music, Reynolds coined the term liminal class, defined as the upper-working class and lower-middle-class. This is a group he credits with "a lot of music energy".

Reynolds has also written extensively about drug culture and its relationship to and effect on music. In his book, Generation Ecstasy, Reynolds traces the effects of drugs on the ups and downs of the rave scene. His evidence of his interest in the topic can be found in Generation Ecstasy, and in his review of Trainspotting, among other things.

Reynolds was influenced by philosophers as well as music theorists, including:

• Gilles Deleuze
• Félix Guattari
• Brian Eno
• Joe Carducci
• the Situationists

He has on occasion used the Marxist concepts of commodity fetishism and false consciousness to describe attitudes prevalent in hip hop music.