Born in Belfast, McKernan was a member of the City of Lisburn Athletics Club. She finished 18th in qualifying at the 1988 Seoul Olympics, before going on to win a silver medal at the 1990 Commonwealth Games representing Northern Ireland, with a throw of 54.86 metres. She then finished 28th in qualifying at the 1992 Barcelona Olympics.
McKernan's personal best throw is 60.72 metres, achieved when winning the silver medal at the Universiade on 17 July 1993 in Buffalo. This is the Northern Irish record, and places her sixth on the UK all-time list, behind Meg Ritchie, Jade Lally, Venissa Head, Philippa Roles and Shelley Newman.[2] She finished 20th in the qualifying round at the 1996 Atlanta Olympics with 58.88 metres (her best throw at an Olympics).