Lake Hennessey is a reservoir in the Vaca Mountains, east of St. Helena and the Napa Valley. The reservoir is formed by Conn Creek Dam, built in 1948 across Conn Creek. The lake was named after Edwin R. Hennessey. Hennessey was a local civic leader who played a role in the development of the Conn Valley reservoir.
The general public is permitted to enjoy the sports of boating and fishing at Lake Hennessey.