The series was originally developed by creator Chris Lang under the working title '27 Arlington Crescent' before being renamed Unforgotten.

Filming for the first series began in March 2015 and lasted twelve weeks, with locations including Liverpool, the London suburbs, Kingston upon Thames, the Essex coast, Westminster and the Fens.

For the second series, filming took place by the River Lea, in the Cotswolds, and along the promenade in Brighton.

The third series used Lymington in Hampshire to stand in for the fictional town of Middenham and its estuary, with an empty mansion at Bulstrode Park near Gerrards Cross serving as the police station and lab; the series began broadcasting only seven weeks after filming wrapped.

Production of the fourth series was delayed, with ITV only confirming in January 2020 that filming had commenced, and filming later recommencing in September 2020 before the series aired in 2021.

The show's opening and closing credits are accompanied by the song 'All We Do', written and performed by the English duo Oh Wonder (Ant West and Josephine Vander Gucht).

Unforgotten has received critical acclaim, with Tom Courtenay winning the 2016 BAFTA TV Award for Best Supporting Actor for the first series and Mark Bonnar winning the 2017 BAFTA Scotland for Best Actor in Television for the second series.

In 2017, an American remake titled 'Suspects' was reported to be in development for ABC, produced by Josh Berman, Sony Pictures Television and BBC Worldwide Productions, featuring a new cast and crew without creator Chris Lang or star Nicola Walker.