Park Hill Neighborhood Improvement Association is the oldest continually active neighborhood association in Columbia. The Association is composed of the owners of lots in Park Hill Additions 1, 2 and 3. In 1922, Judge John A. Stewart first advertised these lots for sale, proposing that these additions would "... make Park Hill to Columbia what the Country Club district is to Kansas City." An equal interest in the 17-acre park (now Stewart Park) was deeded to each purchaser of a Park Hill lot, this interest to reside in the lot itself.

Following the death of Judge Stewart in the late 1930's, certain individuals claimed to have secured title to all park land through foreclosure of an old mortgage and proposed to sub-divide the park into a 60-lot subdivision. The Park Hill Improvement Association, incorporated in June 1937, represented all homeowners in Additions 1, 2 and 3 against this proposal. In a legal battle that was to last until 1947 and was ultimately decided by the Missouri Supreme Court, homeowners defended their stewardship of the property. This battle cost Association residents $3000 (a substantial sum for the time), a debt which was not paid off fully until 1950.

The efforts of PHNIA ensured that Stewart Park, the largest private park in Columbia, is here to enjoy today.