After thanking her supporters, she promised to work hard and to repay the faith party officials had shown by selecting her. She also paid homage to Ms Baker-Hogan in what she said was a good campaign.
Ms Baker-Hogan was generous in defeat, calling Ms Lewis "the bright future of Labour".
"It will be a hard job for her, but I'll be right there behind her and will do my best to support her," Ms Baker-Hogan said.
Ms Lewis said the Whanganui electorate was winnable for Labour and pointed to recent polls which showed Labour had made good gains in party support.
"Party votes carry over to candidate votes. I have an incredible campaign team in Whanganui and in South Taranaki, and with Chester leaving I believe the seat is within our grasp."
Mr Borrows last won the electorate with a 4500-vote majority.
Labour's Whanganui branch chairman Craig Paynter said South Taranaki would be the battleground where the seat would be won or lost.
"South Taranaki has always been a harder area for the Labour Party to win votes. In Whanganui we typically win about 26 of the 28 booths, but in South Taranaki it's considerably bluer," Mr Paynter said.
"Whanganui is most definitely winnable again, especially when you look at the fact that National are also going through a change process. In theory they won't have a candidate that's based out of South Taranaki."
In Labour's favour, Ms Lewis does have strong South Taranaki ties - her parents farmed in Waverley and she has worked in the area.
"It is an area that's been Chester's stronghold, so it's nice to have a candidate that's got strong connections up there as well," Mr Paynter said. "She has lived a considerable part of her life growing up there and has worked in the community up there."
Mr Paynter said Ms Lewis was a very smart and capable woman. "She has lived and worked in this area and is in touch with the locals here and the issues that are important to them - and that's what she'll be focusing on, I'm sure."
Ms Lewis was selected in 2004 as Labour's Youth MP for Whanganui and she started a Young Labour branch in Whanganui.