He served as Secretary of the Milwaukee County Park Commission from its inception in 1907 until his retirement in 1941.
As de facto planner for the city of Milwaukee he sought to alleviate the negative effects of congestion and to prevent sprawling development from destroying the natural landscape.
[1] His plan of 1923 began the purchase and development of 84 miles of greenways along the course of streams and rivers, in order to preserve these waterways in a natural state and to provide city dwellers convenient access to the natural environment.
Whitnall was critical of the prevailing urban park ideal of his day, which he described as, "an artificial attempt to create a natural influence.
Whitnall was a charter member of the Socialist Party of America and also a co-founder of the Commonwealth Mutual Savings Bank of Milwaukee, a cooperative enterprise.