The French actress - who won an Oscar for La Vie En Rose - explained to NPR that she studied the English language more seriously after landing Hollywood roles.
"Well, first of all, I love the English language, so it makes it easier [to learn]," she declared.
Cotillard continued: "And also when I was in Los Angeles in 2008, and I met with Michael Mann and he - we did this movie together, I really had to work on my American accent. And I worked for four months, every day, with a dialect coach, and I improved my English a lot.
"I still have a lot of work, but I feel much more comfortable than a few years ago when I got here and I couldn't express myself properly."
Cotillard has landed a variety of high-profile roles in English-language films in recent years, including Nine, Inception and The Dark Knight Rises.
Her current French-language romance film Rust and Bone is now playing in the US and the UK.
The actress revealed earlier this week that she went through a "violent" period of weight loss after giving birth to son Marcel in 2011.
Watch Marion Cotillard discuss Rust and Bone with Digital Spy below: