Raine's character Jenny Lee quit her role as a midwife to begin working in a Marie Curie cancer hospital, and to start a new life with Phillip Worth (Stephen Ashfield).
The voice of mature Jenny (Vanessa Redgrave) will still narrate future series, as it was explained that she remained in contact with the staff at Nonnatus House and that she continued to share their stories with the world.
Raine has exited the show to pursue new acting projects after becoming a major star in the BBC One period drama.
She recently appeared in BBC Two's Line of Duty, and also starred in Doctor Who biopic An Adventure in Space and Time last year.
Raine will soon appear in the new Sky Atlantic drama Fortitude alongside Christopher Eccleston and Michael Gambon.