He was a deputy state attorney general of Pennsylvania from 1837 to 1839.

[1] On December 8, 1869, McKennan was nominated by President Ulysses S. Grant to a new seat on the United States Circuit Courts for the Third Circuit created by 16 Stat.

He was confirmed by the United States Senate on December 22, 1869, and received his commission the same day.

McKennan served in that capacity until his retirement from the bench on January 3, 1891.

[1] McKennan died on October 27, 1893, in Pittsburgh, Pennsylvania.