The Saturdays star — who became a mother around the same time, welcoming her first child in November 2022 — revealed her father was diagnosed with a brain tumour in August 2022 and died just three months later.
King, 35, shared on social media: "This month is Brain Tumour Awareness month. I’ve always tried not to let myself think about losing someone I love, it’s too hard to think about, but last year this fear became a reality.
Read more: How to spot the signs of a brain tumour
"In August my wonderful Dad was diagnosed with a brain tumour and we had to say goodbye to him just 3 months later.
"It’s still very hard to write about, I’ve been putting off writing this post because it’s easier to push it to the back of my mind and try to keep as busy as possible to cover up the heartache."
The Radio 1 DJ shared memories of her father and admitted how hard it was to watch her daughter Annabella grow up never knowing her grandfather.
She said: "I think of him every single day and talk to Annabella about him when we’re winding down in the evening. For anyone who has lost someone, you might agree that the nights are the hardest, and talking about him in the past tense is something I hate doing."
King revealed her father's diagnosis "came completely out of the blue" while out walking the dog with her one morning.
She said: "My family have been utterly heartbroken by what this horrible disease did to Dad and want to do all we can to raise awareness and funds for @thebraintumourcharity, who work to help everyone affected by a brain tumour or brain cancer.
"Our lives will never be the same after losing our beloved Dad and our love and thoughts go out to anyone whose life has also been touched by cancer."
The Missing You singer's sister Laura is running the London Marathon in April in aid of The Brain Tumour Charity.
Her bandmate Una Healy commented: "So beautifully written Mollie, sending you and all of your family so much love."
King - who shares her daughter with 36-year-old cricketer fiance Stuart Broad - co-presents a weekend afternoon show on Radio 1 with Matt Edmondson and last year launched her own show Future Pop on Thursday evenings. She returned to work from maternity leave in February 2023.
The girlband went their separate ways in 2015.
Watch: The Saturdays Una Healy recalls meeting Mollie King at band audition