Tina Michelle O'Brien was born on 7th August 1983 in Rusholme, Manchester, England, daughter of Tina and Steven O'Brien. She is an actress and model, best known for her role as Sarah Platt in the ITV soap Coronation Street from 1999 to 2007 then 2015 to date. Her other main role has been as Bex Fisher in the BBC school drama Waterloo Road, from 2010 to 2011. O'Brien left Trinity Church of England High School with nine GCSEs in 1999. She starred in the drama Children's Ward in 1997, The Cops in 1998 and Clocking Off in 2000, playing the on-screen half-sister of actor Jack P. Shepherd, who later became her on-screen brother David Platt in Coronation Street.

At the age of sixteen, she joined Coronation Street on 31st October 1999, alongside Nikki Sanderson who played her character's best friend Candice Stowe. She took over the role of Sarah-Louise Platt from actress Lynsay King, despite being three years older than her character. Sarah's pregnancy with baby Bethany Platt, won O'Brien Best Storyline at the British Soap Awards. She also won Best Newcomer at the National Television Awards, and the TV Quick Awards. O'Brien revealed in the British press in April 2007 that she would be leaving Coronation Street before the end of the year.

Her first role after leaving Coronation Street, was in the 2007 pantomime at Manchester Opera House, where she played the title character of Cinderella. Her first television role post-Coronation Street, was in the ITV police television drama Blue Murder, filmed whilst four months pregnant, in which she played a character called Amy Kirkland. O'Brien expressed interest in televised dramas and theatre work, although after becoming pregnant, she decided to put her career on hold.

In 2010, O'Brien starred in the sixth series of the BBC school drama Waterloo Road, as Bex Fisher, the eldest daughter of the new headmistress Karen Fisher. In 2010, she also appeared in The Secret Diaries of Miss Anne Lister, in which she played the character of Miss Browne, a role which required her first on-screen lesbian kiss.
Later in 2010, she appeared in new television drama The Accused, written by Jimmy McGovern, as well as starring in the latest series of Strictly Come Dancing. Her partner was Jared Murillo but she was unable to perform in the third week due to illness. She was eliminated in the 5th week of the competition, after failing to impress the viewers with her Argentine Tango. She fell into the bottom two alongside Felicity Kendal.