In celebration of her landmark milestone, the singer shared a series of photographs on Instagram.
The first three pictures show the 35-year-old posing in a mismatched bikini, accessorising with several necklaces and a bangle bracelet.
The fourth image depicts Allen at a time prior to her sobriety when she appears to have been inebriated.
In the caption for the Instagram post, the “Smile” singer expressed the gratitude she feels for being healthy and happy at this stage in her life.
“One year completely sober!” she wrote. ”So grateful for my health and happiness.”
Several of the musician’s 1.2m Instagram followers shared messages of support for Allen in light of her achievement.
“You look just gorgeous honey! Glowing glowing from the inside out. Very proud of you,” chef Andi Oliver commented.
“Amazing,” said magician Dynamo, while journalist Bryony Gordon remarked: “Congratulations Lily, that’s so awesome!”
Allen has spoken in the past about her former use of drugs and alcohol, telling GQ in 2018 that she “always had been a drug taker”.
“The drinking was an accessory to doing drugs. I realised, ‘Oh, f***, I haven’t taken any drugs in ages, but I’m drinking a lot.’ I was drinking a bottle of Grey Goose a day. It was really bad,” she recalled.
She explained that at one point in her life when she was promoting her music, she would drink early in the morning to get “into work mode for photoshoots”.
In one of Allen’s most recent Instagram posts, she revealed she is currently on holiday in Italy on her own.
“The only downside to travelling alone is the photo opportunities are limited,” she said in a caption that accompanied a selfie taken by the sea.
If you are concerned about your drinking or someone else’s, you can contact Drinkline by calling 0300 123 1100. The helpline is open from 9am to 8pm Monday to Friday, and from 11am to 4pm at the weekends.