2014-2018: Birth and early life
Millie was born on 30th September 2014 to Andrea and Jamie Tate. However, what Jamie didn't know was that Andrea wasn't completely certain of Millie's true parentage as she had a fling with Graham Foster in the time frame of Millie's conception. Despite this, Andrea was sure Millie was Jamie's and concealed the truth of her infidelity.
2019-: Arrival in Emmerdale and true paternity
Millie and Andrea arrived at Emmerdale in June 2019, and soon run into and Jamie's mother, Kim Tate, who gave Andrea a safety lesson after her horse was spooked by Andrea's car. Kim was shocked to learn Andrea and Millie were Jamie's wife and "child" and reluctantly allowed the family to stay with her at Home Farm. It was later discovered that Graham Foster had paid Andrea several years earlier to keep an eye on Jamie, whilst Kim was serving six years in prison. In contrast to Andrea and Jamie, Millie liked Kim and grew fond of her, often calling her "granny".
In late August 2019, Jamie and Andrea worry as they have been unable to obtain a place for Millie at Connelton Primary School and no other school is as close. Kim then suggests Millie could go to Baswick Green Prep School but Jamie makes it clear that he doesn't want his daughter to attend a private school. However, the following day, Kim offers to babysit Millie so Jamie and Andrea can look at another school but secretly arranges a meeting with the school's headmaster, Eustace Briggs. Later, when Jamie returns to Home Farm to find Kim having a meeting with Mr Briggs he is furious. Despite this Andrea convinces him that it's a good offer and that he shouldn't let his own bad experiences come between Millie getting a good education.
On 23rd September 2019, Millie gets injured by one of Kim's horses and is taken to hospital. She recovers but Graham gets suspicious upon hearing Millie's date of birth and becomes convinced she is his daughter following his and Andrea's one night stand. In November, Graham tells Kim the truth and despite Andrea's insistence that Millie must be Jamie's, Kim decided to do a DNA test to determine who Millie's father really is. However, after Andrea suffered a miscarriage on 2nd December 2019, Kim took pity on her and threw the DNA results away without looking, stating that the truth should never be known. However, Graham could not face not knowing if she was his daughter or not and decided to conduct his own test, stealing a strand of her hair without her parents' knowing. On Christmas Day, Graham got back the DNA results and Millie turns out to have been Jamie's all along.
In early 2020, Millie struggled as her parents grew further and further apart and she didn't understand why. In January, Millie was not happy when Kim told Andrea to move out of Home Farm and Millie started to notice how her parents were always fighting. However, in March 2020, Andrea and Jamie decided to give their marriage another go and moved in together with Millie at Tug Ghyll.