Karen Jennings told HSJ that being a Labour candidate will be no barrier to speaking out against the party's health policies.
Unison has been vociferously opposed to the use of private finance initiatives and the establishment of foundation trusts.
Ms Jennings said: 'I know there will come a time when Unison's views might be different from those of the constituency. That's something I will have to cross as I go along.
'I think the reason I was chosen was because I wasn't toeing the party line and am an independent thinker.
'I will have to wear two hats - one for Unison and one for Labour.'
Ms Jennings will be standing in Hornsey and Wood Green in London, which is currently held by Liberal Democrat MP Lynne Featherstone who won the seat from Labour in 2005.
Ms Jennings was selected from a list of 13 candidates.
Explaining her motives, she said: 'They needed a political heavyweight and an experienced campaigner, and fingers pointed at me.
'I was honoured and it got me thinking I might be the right person.'
She added: 'I will be looking to influence health policy, which would mean, if I were successful, joining the health select committee and trade union group in Parliament.
'I am a moderniser and want to see reforms developed but also to articulate concerns about those developments.
'The pace of change has been too fast - policy developments need to be based on evidence.'