Born in Providence, Rhode Island, Knowles received an Artium Baccalaureus degree from Brown University in 1836 and a Bachelor of Laws from Harvard Law School in 1838.

He was in private practice in Providence from 1838 to 1855, and was a member of the Rhode Island General Assembly in 1855.

He was returned to the Rhode Island General Assembly in 1866, and was a city solicitor in Providence from 1866 to 1867, thereafter making a final return to private practice until 1869.

[1] Knowles received a recess appointment from President Ulysses S. Grant on October 9, 1869, to a seat on the United States District Court for the District of Rhode Island vacated by Judge J. Russell Bullock.

He was confirmed by the United States Senate on January 24, 1870, and received his commission the same day.