The 30-year-old welcomed her first child with husband Hugo Taylor in May.
She took to Instagram to share a snap of her post partum body, telling fans: "I’m in awe of what my body has achieved over the last year."
The photo shows Millie holding Sienna in a selfie as she poses in her bedroom in underwear.
In the post, the former Made In Chelsea star took aim at vicious trolls who commented on her appearance.
She wrote: "With Sienna nearing 10 weeks old nothing has given me more purpose than knowing that a small life depends entirely on me to feed and nourish her as she grows.
"I try to live a healthy, balanced lifestyle and I count myself lucky that throughout my adult life I’ve felt and identified as pretty body confident.
"But now all that has changed along with every other aspect of my life, which makes me look at my body in a totally new light and it’s been a big adjustment!
"Fitting into my pre pregnancy clothes seems a long way off and I have no intention of rushing into ‘getting my body back’ but it would be a lie if I said I feel totally happy and at one with my body post pregnancy.
"The process has been made a lot harder by some overly strong and hurtful comments online about my appearance (as much as you try and ignore them).
"I’m in awe of what my body has achieved over the last year but I find it hard not seeing the girl I know to be me looking back at me in the mirror."
Millie went on to reveal how she was often too critical of her figure before giving birth.
The new mum continued: "I now see photos of myself pre-pregnancy and remember thinking at the time I didn’t like my body at certain angle but now I look back and think I looked great!
"It’s made me realise how critical I am of myself and how it’s so crucial, as women, that we learn to let ourselves off the hook. My mantra has always been that you exercise because you love your body not because you hate it, so any movement I do now is with the intention to feel good and never to punish myself."
Millie concluded the post, saying: "Sienna is worth every extra inch, pound and stretch mark. My body made me a Mother and for that I'll always be grateful 💗 #postpartumbody."
Praise for new mum Millie Mackintosh
The honest post certainly didn't go unnoticed by followers, who rushed to comment their support.
A fan said: "Finally a celeb post body that resembles the rest of us mere mortals. You look incredible, a super fit, strong mumma. How your body is supposed to look 10weeks later... #positiverolemodel well done."
Another praised the star, saying: "You look amazing and this is a powerful inspirational post. Thank you for your bravery for posting this you look beautiful."
While Vicky Pattison commented: "This is such a powerful post 😍 ."
When did Millie Mackintosh give birth?
Millie and her husband Hugo, 34, welcomed baby Sienna into the world in May.
The proud new parents revealed to Hello! magazine: "We are delighted to announce the arrival of our darling girl who arrived on Friday 1 May at 1:21pm, weighing a very healthy seven pounds.
"We are eternally grateful to the doctors, nurses and midwives for taking such good care of us.
"Mum and baby are both doing incredibly well and we are looking forward to bringing our daughter home and spending time together as a family."
Following Sienna's birth, Millie returned to social media by sharing an adorable picture of her new family on Instagram.
She announced her pregnancy joy back in November, when she revealed she was looking forward to welcoming a baby girl.
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