Mendes directed his wife in the award-winning movie opposite Leonardo DiCaprio, which was about a couple going through a stormy break-up.
During the promotion of the film last year, Kate admitted that she'd found it awkward being directed by her husband - particularly during sex scenes with her former Titanic co-star.
'I just kept saying, "This is too weird,"' she said in an interview last year. 'Leo told me to get over it, but I said, "You're my best friend and he's my husband. This is definitely a bit weird."'
Kate and Sam, who were married back in 2003, reportedly split earlier during the year, but released a joint statement yesterday, confirming the separation.
'The split is entirely amicable and is by mutual agreement,' it read. 'Both parties are fully committed to the future joint parenting of their children.'
A friend of the actress has said that she now plans to divorce Mendes, saying: 'That's it, it is final.'
'Kate is devastated. She thought their marriage was forever, and now she has two children from two marriages and both are not living with their fathers.'
Kate is currently in Mexico with her two children, Joe, six and Mia, nine, who was from her previous marriage to assistant director Jim Threapleton.
Sam Mendes is currently in the US, where he is working on a couple of Shakespeare theatre productions.
'He became bored working on his latest theatrical project and he took that boredom home with him,' says a source close to the director.
'That led to the spark being taken out of their marriage.'