The couple, who began dating after meeting at 'Revenge' star's Ashley Madekwe's wedding in June 2012, called timed on their romance "a few months" ago.
A source close to the 27-year-old actress told E! News: "It was amicable. She is concentrating on her new projects and is in production on a new film."
The couple sparked rumours they were engaged earlier this year when the former 'O.C.' beauty was spotted wearing a ring at the Coachella Music Festival but her publicist insisted at the time she was "definitely not engaged".
Mischa has thrown herself into her work since the split and is currently filming the thriller 'Beyond Justice' with Vinnie Jones and Danny Trejo.
The actress previously praised Sebastian for supporting her career and attending public appearances with her.
She said: "He's great. I was like, 'You don't have to come to the store opening, you don't have to do anything.' But he said, 'No I'm coming to support you.' "
Mischa previously dated Kooks singer Luke Pritchard and Whitestarr performer Cisco Adler.