Iles was born in Dunstable, Bedfordshire, her parents both musicians (her father a drummer and her mother a pianist). She started her musical education at primary school, where she learnt to play the harmonica and the clarinet, and at eleven years old she won a junior exhibition at the Royal Academy of Music, where she studied clarinet and piano (1974–1981). She became a member of the Bedfordshire Youth Jazz Orchestra. As a result of her increasing involvement with jazz, she went on to the Leeds College of Music to study alto saxophone and piano (1981–1984).
She joined former husband Richard Iles' group Emanon (R. Iles trumpet, Mike Walker guitar, Iain Dixon saxophone). The members were also all founder members of the Creative Jazz Orchestra, alongside Vince Mendoza, Anthony Braxton, Mike Gibbs, Kenny Wheeler and Mark Anthony Turnage. Iles also began playing with several London-based bands, such as Steve Argüelles' group, Mick Hutton's Straight Face, Stan Sulzmann's quartet, and Tina May.
Iles won the 1996 John Dankworth Special Award at the BT Jazz Festival.
Iles is senior lecturer at Middlesex University, and has taught at the University of York, Leeds College of Music, the Guildhall School of Music, and in Bulgaria, Holland, France, and Finland. She also runs training workshops for the Associated Board (Jazz syllabus and is a tutor for the National Youth Jazz Collective.
- 1992: Pyrotechnics — Sylvan Richardson with Mike Walker and Julian Arguelles (Blue Note)
- 1994: By The Way — Mike Gibbs Orchestra (Ah Hum)
- 1994: Treasure Trove — Stan Sulzmann/Nikki Iles (ASC)
- 1997: The Tan Tien — Martin Speake/Nikki Iles (FMR)
- 1998: Change of Sky — Tina May/Nikki Iles (33 Jazz)
- 1998: Snap — Foolish Hearts (Iles, Steve Berry, Paul Clarvis, and Anthony Kerr) (rfm101)
- 1999: Secret Moves — Dick Walter/Jazz Craft Ensemble (ASC CD29)
- 1999: Don't Ask — Geoff Simkins quartet (Simkins, Iles, Simon Woolf, and Martin France) (Symbol SR19991201)
- 1999: One Fine Day — Tina May Group (May, Iles, Alan Barnes, and Mick Hutton) (33Jazz 50)
- 2000: Secret Anglo-Canadian Project — Iles, Martin Speake, Anthony Michelli, and Duncan Hopkins (Basho Records SR CD3-2)
- 2001: Some Times — Ingrid Laubrock Group (Candid CCD7977)
- 2003: Romancing — Scott Hamilton and The Tina May Quartet (Linn Records AKD202)
- 2003: Veils — Nikki Iles Quintet (Symbol)
- 2004: Everything I Love Nikki Iles Trio (Basho Records SRCD 5-2)
- 2004: More Than You Know, with Tina May and Tony Coe (33 Records, 33 Jazz 100)
- 2006: A Wing and a Prayer, with Tina May and Stan Sulzmann (33 Jazz, 33JAZZ134)
- 2012: Hush, with Rufus Reid and Jeff Williams, (Basho, SRCD 38-2)