Amenity demo (2002)
In 2002, Westerholt formed Delain, after he was forced to leave Within Temptation due to Pfeiffer's disease. In the same year, they independently released the demo Amenity. After the demo, Westerholt temporarily left the project behind but continued to write music.
In 2005, he joined with Charlotte Wessels, and was signed to Roadrunner Records. At the time, the band was envisioned as a musical project featuring a host of guest musicians, and not as a live band.
Lucidity brought to fruition Westerholt's dream of a musical project, featuring many well-known musicians from the metal community: Marco Hietala from Nightwish and Tarot (bass, vocals), Liv Kristine from Leaves' Eyes (vocals), Ariën van Weesenbeek from ex-God Dethroned, Epica (drums), Ad Sluijter from ex-Epica (guitar), George Oosthoek (growling) and Guus Eikens, both former members of Orphanage, Sharon den Adel from Within Temptation (vocals) and Jan Yrlund from Imperia (guitar).
The vocalist, Charlotte Wessels, during a concert in 2006.
Despite many delays, Lucidity was released in September 2006, to generally positive reviews. The album spawned three singles: Frozen, See Me In Shadow and
The Gathering, and the response to the album allowed Delain to tour in support in the Netherlands and neighbouring countries. Rob van der Loo, Ronald Landa and Sander Zoer were added on bass, guitar and drums respectively to round out the live band.
April Rain (2008-2010)
At the end of 2007, Delain announced that they were working on their second album. New songs from the album – "Stay Forever" and "Start Swimming" – were added to the band's setlist in early 2008. Their second album, April Rain was released on February 9, 2009. The lead single was the title track, "April Rain", followed by "Stay Forever". The band toured extensively in support of the album, visiting many new countries such as the United States and Mexico, as well as performing at high-profile festivals in Europe, including Hellfest, Lowlands, Wacken Open Air and Sonisphere. In February 2010, Delain announced they had started working on their third album, and that bassist Rob van der Loo would leave the band the same year due to time constraints. In March, Otto Schimmelpenninck van der Oije was announced as Rob's successor.
On October 4, 2010, Delain announced they were to part company with guitarist Ewout Pieters. Timo Somers was announced as his replacement on April 17, 2011.
We Are the Others (2011–2012)
During their 2011 live shows, Delain debuted three new songs — "Manson" (later renamed to "Mother Machine"), "Get the Devil Out of Me" and "Milk and Honey" — from their third album, We Are the Others.
In an interview with Sonic Cathedral, Wessels discussed the inspiration she drew from the Sophie Lancaster case:
- I remember when I first heard about it … it wasn’t on Dutch news, I just heard about it through Internet networks and the goth scene … there was this movie made about it, a short film of about four minutes. I saw it, and I just cried. It’s so incredibly sad! After seeing the movie, I didn’t really DO anything with it until we were working on “We Are The Others.” But the basic idea of the lyrics was there. It was just supposed to be a song about “we are the others” and a feeling of togetherness. On the one hand, being proud of whoever you are, whether you divert from the norm in whatever way you divert from the norm. But on the other hand, it is also kind of a song for “others”. We just wanted a song about acceptance.
Originally intended for release in early 2012, the album's release date was unknown due to Warner Music's purchase of Roadrunner Records. However, it was announced via a Facebook message on the band official page that the album would be released on June 1, 2012 with the album's first single, 'Get the Devil Out of Me' being released on 13 April 2012  via CNR Music, who took over Delain from Warner Music. The second single from the album was the title track, 'We Are the Others'.
It was announced on 3 November that Delain would headline Dames of Darkness Festival (Wolverhampton, UK) which they had previously headlined in 2009 (under the Femme Metal Festival).
On 26 February 2013, Delain announced that they will release a special album entitled Interlude that would include new songs, covers, live tracks, special versions of previous songs and a bonus DVD. It was released on various dates throughout the first week of May worldwide.
The Human Contradiction (2014–2015)
Charlotte Wessels and Timo Somers, during a 2015 festival in Germany.
On 23 January 2014, Delain announced that they would release a new album entitled The Human Contradiction. It was released on April 5, 2014 in Europe, and on April 8, 2014 in North America.
On June 4, 2014, it was announced that drummer Sander Zoer would step down from the band due to personal reasons; he would continue working behind the scenes of Delain. The replacement drummer was announced to be Ruben Israel, who had stepped in for Zoer during Delain's tour with Kamelot. Zoer continued to perform until September, until he left for personal reasons and was replaced by Israel.
The band brought with them Merel Bechtold (MaYaN, Purest of Pain) as a second guitarist for their UK tour in November 2014. Bassist Otto Schimmelpenninck van der Oije suffered an injury on November 26, 2014, when he was accidentally hit by a streamer cannon, causing his left testicle to rupture. This incident was said to be caused by uncertain stage positioning, as there were six people onstage instead of the usual five. While he was recovering, the band used pre-recorded bass parts. No permanent damage was done, and as of January 12, 2015, Schimmelpenninck van der Oije has resumed touring. Merel Bechtold filled in for guitarist Timo Somers until February, due to Somers' full schedule.
Delain toured with Sabaton and Battle Beast, and also North America with Sabaton as support for Nightwish for the first half of 2015 and returned to Latin America during October as support of them. Delain toured as support again for Nightwish's tour on the second North American leg in 2016 with Sonata Arctica. This makes it the fourth year in a row the band has toured in the United States and Canada.
On October 19, 2015, Delain made recurring touring guitarist Merel Bechtold a permanent member.
Lunar Prelude EP, Moonbathers (2016-2018)
Delain announced an eight track EP of new and live songs named Lunar Prelude. The EP was released on February 19, 2016. Their first single, Suckerpunch, of the EP was released on February 5.
The band announced a new album of ten original songs and one "Queen" cover named Moonbathers. The record was released on August 26, 2016.
Delain's first live video album, A Decade of Delain: Live at Paradiso was filmed at the Paradiso in Amsterdam, Netherlands for the band's 10th anniversary on December 10, 2016. It was released on October 27, 2017.
On August 30, 2018, Joey de Boer was made a permanent member of Delain after filling in for Ruben Israel, who left the band in October 2017.
Hunter's Moon EP (2018-present)
Delain announced a new EP titled Hunter's Moon on December 13, 2018. The EP will be released on February 22, 2019.
Masters of Destiny, the first single off of the EP was released on January 11, 2019 along with a music video.