Mia Sasha was raised in Sidcup in the London Borough of Bexley and lived there with her mother, Angela McKenna Bruce, father and two younger sisters, Ellis and Anya, who is also an actress.
Mia attended Sapphire Dance Academy in Bexleyheath, and attended Chislehurst and Sidcup Grammar School until she moved away to the historic market town of West Malling in Kent in 2010. Once she had moved, she then attended Maidstone Grammar School for Girls for the remainder of her educational life.
Mia's first role was a ballet girl in the West End production of Billy Elliot. Her first TV role was the minor role of Ester in the TV short, Small Dark Places. She then starred in Holby City as Abi Taylor for one episode. Mia then appeared in BBC's soap opera drama, EastEnders as Penny Branning, Jack Branning's wheelchair-bounded daughter. She had a two episode guest appearance in January 2008, and then another three episode guest appearance stint in April 2008. Mia next starred in The Bill in 2009 as Keira Curtis for one episode. She then moved on to Doctors for three episodes.
Later in 2009, McKenna-Bruce starred in her first film role, The Fourth Kind, playing Ashley Tyler. Her next role was as main character Tee Taylor in Tracy Beaker Returns for three years from 2010 to 2012. She auditioned for the role in 2009 at 11 years old. In 2011, Mia appeared in the TV series Spy for 1 episode and in 5 episodes of The Tracy Beaker Survival Files. Mia reprised the role of Tee Taylor in The Dumping Ground for 4 series from 2013-16. She made two guest appearances in the fifth series' two-part finale in 2017. Mia, again, reprised her role as Tee Taylor in The Dumping Ground Survival Files in 2014, and The Dumping Ground: I'm... (a webisode series) in 2016.
In 2016, Mia was then cast in the upcoming show The Marshlands. Mia guest starred in another episode of Holby City as Asia Lucas, as well as guest starring in BBC Three's Josh. Mia was then cast as the lead in a feature film, The Rebels.
Mia is currently dating Tom Leach, a fellow young actor. Mia previously dated actor Ceallach Spellman for a number of years, though the two remain close friends.