The name stems from the local Aboriginal name of "Kincumba" meaning "towards the rising sun" or "to tomorrow". A large open flat rock area at the highest plateau of Kincumba Mountain, which commands panoramic views of the region, was used by Kuringgai people as a meeting place and camp. Being the highest open position in the region, it was first to receive the morning sun as it broke over the eastern horizon Tasman Sea/Pacific Ocean, and so this area was also referred to as "towards tomorrow". Signs of habitation in the Kincumber area include axe-grinding grooves and rock engravings on Kincumba Mountain. Due to the freshwater creek, water access from Brisbane Water and the Hawkesbury River, and an abundance of old rainforest timber in the surrounding valley hills, Kincumber was one of the earliest settlements on the Central Coast, supplying wood for the needs of the growing colony at Sydney in the early 19th century.
Boat building became an early local secondary industry after timber-getting, to meet the primary necessity of water transport. Boat-building along Kincumber Creek in Kincumber continued until the early 21st century.
Kincumber Was once home to a Monazite processing plant that processed mineral sands that were mined in the local area, The plant was operated on the edge of Kincumber industrial estate, and backed on to Kincumber creek, the site "lot500" now abandoned still retains higher than average levels of radiation and has been deemed unsafe for development.
In the mid 1980s there was no local shopping center, and by the late 1980s the local shopping center was funded by a former world tennis player John Newcombe.The 1980s also brought infamy to Kincumber, as the town made international headlines for the mistreatment of Eve van Grafhorst, a girl who contracted HIV through an infected blood transfusion, which was her last of eleven after being born prematurely. Grafhorst and her family eventually were forced to move to Hastings, New Zealand by the people of the town, after Grafhorst bit a child at kindergarten and wasn't allowed to return unless she wore a face mask in 1985. Soon after, the family were told by the Gosford City Council and childcare centre head: "Withdraw your daughter from kindergarten or we will go public with the fact she has HIV". Grafhorst and her family lived a relatively normal life in Hastings, where she was even allowed to attend school. Eve Van Grofhorst eventually would lose her fight to the disease, aged only eleven in 1993.
At a federal level, Kincumber is within the Division of Robertson. Previously held on a safe margin by Jim Lloyd of the Liberal Party, the seat changed hands in the 2007 Federal Election, and was held by the Australian Labor Party's Belinda Neal. In 2010 the seat was won by Labour member Deborah O'Neill.