Tsawwassen First Nation Chief Kim Baird has lost her seat by nine votes to a young challenger in a surprising election defeat on Wednesday night.

Baird was defeated by Bryce Williams, the youngest member of Tsawwassen's executive council, in the vote by 148 of the band's 260 eligible members. Williams is in his early 20's and previously served one term on the executive council.

Baird had been chief of the Tsawwassen First Nation for 16 years and during her six terms she negotiated B.C.'s first urban aboriginal treaty, which helped create a new governance structure that replaced the Indian Act and saw the band get legal title to several parcels of land in Tsawwassen in 2007.

Tsawwassen First Nation Chief Kim Baird led her band through a new treaty and a massive development plan. (Evan Mutsui/CBC)

Since then she spearheaded a series of multi-million dollar deals to build one of B.C.'s largest shopping malls, a container shipment centre and new housing for thousands on some of that land.

Baird said she was keen to return as chief and not entirely sure why she lost the band's support.

"It's hard to know for sure, it was a pretty quiet campaign. People weren't saying a whole lot. Bryce's campaign wasn't contrary to what I was doing necessarily, so I think at some point people just want a change in leadership," she said.

Premier Gordon Campbell and Tsawwassen First Nation Chief Kim Baird celebrate the first modern treaty negotiated under the British Columbia Treaty Commission process, in April, 2009. (B.C. Government)

She will no longer serve in the band's 13-member legislature after she steps down on Sept. 16.

She described her replacement Williams as a carver with a passionate belief in First Nations culture and a capable leader, noting she was also first elected as chief in her 20s.