Joyner trained at the Royal Welsh College of Music and Drama, since arriving on EastEnders in June 2006, Joyner as Tanya Branning has been at the heart of the action in Albert Square and has had a wide scope of storylines; the episode in which her character Tanya discovered her husband Max Branning's affair with Stacey Slater was watched by 13.9 million people. It was the highest rated television broadcast of 2007 and went on to be nominated for a BAFTA Award and Jo, the storyline and the episode were awarded at the 2008 British Soap Awards.
Before starring in EastEnders, her most noted role was as Beth Nicholls in the popular nursing drama No Angels. She had also starred as Lynda Moss, in two episodes of Doctor Who ("Bad Wolf" and "The Parting of the Ways", BBC One, June 2005). Her character also featured in a flashback in the episode Journey's End.
Joyner has also appeared in the feature-length drama Night Flight, and on television in Fat Friends, Silent Witness, Clocking Off, the 2004 BBC adaptation of North and South and as one of the stars of the Five comedy sketch show Swinging (2005). For her performances in the latter Jo Joyner won a Rose d'Or international award for 'Best Female Comedy Performer'.
She also featured briefly in a sub-plot of the two-part drama The Second Coming (ITV1, 2003). Her character has a bigger role in cut scenes that feature as extras on the DVD. Also in 2003, Joyner appeared in Series Two, Episode Five of Spooks, as a firefighter named Stephanie Mills.
After taking A-levels at Stratford-upon-Avon College, Joyner took a degree in English Literature and Theatre Studies at the Welsh College of Music and Drama in Cardiff. She graduated with a BA Hons and the year Prize for Acting.
Joyner played 'Lex', a violent girl gang member in the award winning play 'Raw' by Chris O'Connell, produced by Theatre Absolute at the Edinburgh Festival in 2001 and in the subsequent National Tour. She also played a male role in Brendan Behan's play The Hostage and appeared in The One With the Oven, at the Royal Court Theatre.
Joyner was born in Essex but grew up near Banbury, Oxfordshire. Her parents are Anne and Peter; she has an older brother, Dan. Joyner lives in London with her husband, actor Neil Madden.They married on 14 July 2007 and currently live in London. In 2008, she told internet users that she did not have any Facebook, MySpace or Bebo accounts when it was revealed that people were posing as her online and posting false information. Joyner announced in July 2009 that she was pregnant with twins as a result of IVF treatment. She gave birth to a boy, Freddie and a girl, Edie, on 7 December 2009.