He was in private practice in Hastings from 1875 to 1888, also serving as a prosecuting attorney of Barry County, Michigan from 1879 to 1883.

[1] He was in private practice in Grand Rapids, Michigan from 1888 to 1906 with the law firm of Knappen, Uhl & Bryant, which had been founded as Fletcher & Wanty, and which continues to exist today as Wheeler Upham.

[2] Knappen was nominated by President Theodore Roosevelt on December 3, 1906, to a seat on the United States District Court for the Western District of Michigan vacated by the death of Judge George P. Wanty, who had founded the law firm of which Knappen was a partner.

[2] He was confirmed by the United States Senate on December 10, 1906, and received his commission the same day.

His service terminated on February 8, 1910, due to his elevation to the Sixth Circuit.