The 27-year-old The Wanted singer told Hello! magazine about the 'emotional' proposal and how he plans to invite Justin Beiber to the wedding.
The pop star, who was recently a finalist in Channel 4's The Jump, began dating Kelsey in 2009.
Tom explained that it took him three months to plan the proposal that has made him feel 'like the happiest man in the world.'
'There was a trail of rose petals leading to the TV where I'd set up a montage of all our pictures from the day we met until now.
'I was crying my eyes out at this point. Then at the end it said - here's to the next chapter, turn around - and when she turned around I was down on one knee.'
Despite planning every last detail, Tom didn't account for his own emotions; 'I literally could not get my words out, I was so overcome with emotion.'
To make the moment as personal as possible, Tom recruited the help of a musician friend to pen a song for the couple, which was playing in the background during the proposal.
Actress Kelsey, who has starred in Harry Potter and the Prisoner of Azkaban, The Interceptor and EastEnders, took to her Twitter account to confirm the news.
Yay! I can finally tell you all.. @TomParker proposed to me.. He liked it and put a ring on it (about time ð) ð’ð’ð’ let's plan a wedding xx— Kelsey Hardwickð’•ð¦ (@Being_Kelsey) March 21, 2016
Although the couple haven't yet set a date to walk down the aisle, they've already drawn up the guest list.
Tom explained how he'll be inviting the Beckham family, who he first met when The Wanted were invited to perform at Romeo's 10th birthday in 2012.
'I love David, Victoria and the kids.'
They also want Justin Beiber, who shares the same record label as the band, to come along on the big day.
Kelsey has her own celebrity pals, including Essex-born singer Pixie Lott who was present on the night the pair met seven years ago; 'I'm dying to tell her we're engaged, she'll be so happy!'
Tom wants his bandmates to be 'ushers' on the big day too, and has special plans for Jay McGuiness - who won Strictly Come Dancing - choreographing their first dance.
Jay took to Twitter to reveal his thoughts on the responsibility.
If I choreograph the first dance it will include Tom humping the floor as Kelsey does ribbon gymnastics, all in total silence.— Jay McGuiness (@JayMcGuiness) March 21, 2016
We can't wait to see it!