She hit the headlines this week after being the latest celebrity to bare her bump in the name of art. The portrait, by British artist Jonathan Yeo, was painted just weeks before she gave birth to daughter Marlowe on July 7.
So in celebration of all things Miller, we've found ten facts about the star.
1. Sienna Rose Miller was born in New York on December 28, 1981, but was raised in London. Her mother, Josephine, was a South African model, her father, Ed, an American banker. She holds both English and American passports, and moved back to America aged 18. She learnt to drive in the US Virgin Islands, because in her words, it was easier to pass there.
2. True to her trade, Sienna was very nearly born in a theatre. Her mother went into labour whilst watching The Nutcracker.
3. Sienna's older sister Savannah is a fashion designer, and the sisters collaborated on fashion label Twenty8Twelve, before stepping down as creative directors earlier this year to concentrate on other projects.
[Sienna Miller and Hayden Christensen in Factory Girl]
4. She has two dogs named Porgy and Bess, named after the two leading characters in George Gershwin's opera of the same name. They are both mongrels, and Sienna bought them from a rescue centre with her then-partner Jude Law.
5. Sienna has (reportedly) been engaged three times, but has never made it down the aisle. Her first attempt was back in 2004 with Alfie co star Jude Law, but it ended in tears when he got cosy with the nanny. Her second engagement, to Rhys Ifans in 2008, ended after just four months amidst rumours she had been getting close to an ex. She was rumoured to have got engaged to current boyfriend Tom Sturridge in January, and the pair had baby Marlowe earlier this year. Third time lucky?
6. She is a regular feature on FHM and Maxim's top 100 sexiest women lists, and was voted #2 in AskMen.com's 'Top 99 Most Desirable Women' in 2006. But one poll position she probably wasn't celebrating was when she and Jude Law were voted the 5th tackiest couple by America's Star magazine in 2005.
7. This isn't the first time Sienna's stripped off for her work. She appeared topless on the 2003 edition of the exclusive Pirelli Calendar, in 2004 comedy Alfie and in the March 2006 issue of Vanity Fair, to name just a few. However her liberal attitudes only stretch so far - she sued both The Sun and the News of the World back in 2010 after they published naked pictures of her on the set of Hippie Hippie Shake.
8. Sienna sparked controversy back in 2007 when she admitted to taking drugs, describing them as "loads of fun". She admitted to experimenting with morphine to "feel what heroin was like", and described magic mushrooms as her "drug of choice".
9. She's a huge fan of Chelsea FC. Growing up near Stamford Bridge, Sienna has said she looks out for their results every weekend. She is also a fan of golf.
10. And finally, to the disbelief of new mothers everywhere, Sienna is now back at work promoting her new HBO drama The Girl, just weeks after giving birth... and not an ounce of baby weight in sight.
Photo gallery - Celebrities naked and pregnant: