Seabeck went to Harold Hill Grammar School (later Upper Bedfords Park School from 1973) in Harold Hill in London. She studied at the North East London Polytechnic (now the University of East London) in Stratford. Seabeck was a member of the MSF (now Amicus), and whilst working at the House of Commons became the secretary of the South Thames Community Branch. She is also a member of the feminist Fawcett Society and the Labour Women's Network.
Seabeck was picked for the Plymouth Devonport seat from an all-women shortlist.
Seabeck was elected at the 2005 general election with a majority of 8,103, replacing David Jamieson. She served as a Government Whip attached to the Department for Business, Enterprise and Regulatory Reform from 2007 to 2008.
In the 2010 general election Seabeck successfully held her seat in Plymouth Moor View.
On 11 October 2010, Seabeck was appointed Shadow Housing Minister, working with Caroline Flint, the then shadow secretary for Communities and Local Government. On 7 October 2011, she was appointed Shadow Defence Minister during Ed Miliband's reshuffle.
In December 2010 it was revealed that Seabeck was under investigation by the Parliamentary Commissioner for Standards in relation to declaration of member's interests. Seabeck was subsequently asked to apologise for speaking in a debate on fire safety without declaring that her husband, MP Nick Raynsford was a member of the Fire Protection Association.
She was also a member of the Public Bill Committee for the Defence Reform Act 2014
In 2014 she tried to introduce an exemption from the so-called 'bedroom tax' so that victims of domestic violence with a "panic room" installed were no longer penalised for the additional room.
In the 2015 UK General Election, Seabeck again stood as the candidate in the Plymouth Moor View constituency, but was defeated.
She married her former husband Denis in 1975 and they have two daughters. She is now the wife of the former MP for Greenwich & Woolwich, Nick Raynsford, for whom she worked as researcher before entering Parliament. Her father was the former Labour MP for Peterborough, Michael Ward. Seabeck hired one of her daughters at very short notice to work for three weeks to cover a staff absence.