The 35-year-old Smile singer began dating the 45-year-old Stranger Things actor in summer 2019 and the pair have filed paperwork to obtain a marriage licence in the Nevada County Clerk’s Office.
The marriage licence, which names the singer as Lily Rose Cooper – her married name with her ex-husband Sam Cooper – records the date of their wedding as Monday, 7 September.
The couple recently enjoyed a family holiday in Croatia with Allen’s two daughters Ethel, 8, and Marnie, 7.
Watch: Lily Allen and David Harbour obtain marriage license
The singer can be seen in recent photos posted on her social media sporting a large diamond ring on her engagement finger.
Last week Harbour revealed on Instagram he was in Las Vegas with Allen and her daughters.
The couple have been living in lockdown together this year.
Previously they were conducting a long-distance relationship across the Atlantic, with Harbour living in New York and Allen in the UK with her children.
Allen married builder Sam Cooper in 2011, shortly after welcoming their first child. The couple announced their split in 2015 and they divorced amicably last year, sharing joint custody of the girls.
The F*** You singer has previously been fiercely protective of her daughter’s privacy, rarely posting pictures of them on social media.
But she recently shared multiple pictures of her daughters and Harbour enjoying time on holiday together in eastern Europe.
Allen confirmed rumours the pair were dating in October 2019 when she posted a picture of Harbour performing at a Saturday Night Live comedy gig on her Instagram stories, which she captioned: “Mine”.
Allen – daughter of actor Keith Allen and sister of Game Of Thrones star Alfie Allen – split from grime MC Medirian Dan in 2018 after three years of dating.
Last year the No Shame singer admitted she had cheated on Cooper during their marriage, paying prostitutes for “lesbian sex romps” while she was away on tour and feeling lonely.
Read more: Lily Allen accused of 'mum shaming'
In July she announced on Instagram she was celebrating one year sober.
Harbour is best known for playing brooding police chief Jim Hopper in Netflix show Stranger Things. He reportedly split from Fantastic Beasts And Where To Find Them actress Alison Sudol, 35, in early 2019.