Kearney's interest in acting began whilst at secondary school. After repeatedly being overlooked for the lead roles in school productions, she decided to join the Liverpool Everyman Youth Theatre. It was there, in 1986, that she was spotted by members of the production team at Mersey Television, an event that resulted in her being asked to audition for a role in Channel 4's Liverpool-set drama series Brookside. Kearney thought she would be little more than an extra, but she actually landed the role of Damon Grant's girlfriend, Debbie McGrath. The character was intended to appear in just a handful of episodes, but Debbie was a great success and the role went on to last for over a year.
When Simon O'Brien decided to leave The Close, he requested that his character be killed off so he wouldn't be tempted to return to the show. Because Damon was so popular, particularly with younger viewers, it was decided to create a 'soap bubble' in which the character would meet his demise — Damon and Debbie became Britain's first mini-series spin-off from a soap opera. A year after filming her final scenes, Kearney was asked to return for a few episodes. Christmas 1988 saw Debbie McGrath make an unexpected return to visit Sheila Grant, Damon's mother; she arrived with her baby, Simon — Damon's son.
In 2005 Kearney landed a role in channel 4 hit Shameless playing the role of Marty's girlfriend Sue. She left the show along with her on-screen partner in 2007. She had a number of TV roles following her departure before landing the role of Jessica Harrison in Casualty, in January 2008.