The reality TV personality made the announcement on social media, sharing a photoshoot of herself and her eldest son Roman, almost two, with her 1.7 million Instagram followers.
In the shot, Mecklenburgh is raising her shirt to reveal her baby bump as little Roman imitates her mum, pulling up his jumper and putting a hand on his belly. She captioned the shot only writing: "Here we go again."
In her Instagram stories, the star shared a second shot of Roman holding up a scan image of the new baby.
Watch: Lucy Mecklenburgh talks about how pregnancy changed her relationship with her body
Mecklenburgh's post blew up amongst her followers, with many fans and fellow celebrities flooding the comments with well-wishes.
Of her TOWIE co-stars, Samantha Faiers wrote: "Awww huge congratulations. Beautiful picture. Baby number 2 xxx"
Courtney Green, Amber Dowding, Chloe Lewis, Jessica Wright and Clelia Theodorou also shared their congratulations.
Made in Chelsea's Binky Felstead and Lucy Watson also appeared in the comments of Mecklenburgh's post alongside Love Island stars Rosie Williams and Kendall Rae Knight and Geordie Shore's Vicky Pattinson.
Corrie's Catherine Tyldesley also shared her excitement, writing "Yeyyyyyyyyyy. Huge congratulations my loves!" while the soap's star Helen Flanagan added: "Ahhhhhhhh congratulations darling lucy and Ryan."
Brooke Levi Vincent and Lucy Fallon also shared some sweet messages as well as Hollyoaks's Nikki Sanderson.
Mecklenburgh and Thomas — also has a daughter Scarlett with his ex Tina O’Brien — have been dating since 2017, after they met on a celebrity version of Bear Grylls' The Island, and have been engaged since 2019, though a wedding remains on the back burner.
Speaking to The Sun, the star joked that they struggle to organise dates, let alone a ceremony.
“Life is different now – there’s more communication but less spontaneity."
Watch: Ashley Graham, pregnant with twins, shares video of her growing baby bump