Background
Dennis was born at "Beverly" in Pocomoke City, Worcester County, Maryland, December 17, 1771. Completed preparatory studies in Washington Academy. Attended Yale College.
Studied law.
Was admitted to the bar in 1793 and commenced practice in Somerset County, Maryland. Served two terms in the Maryland House of Delegates. Elected as a Federalist to the Fifth and to the three succeeding Congresses (March 4, 1797 – March 3, 1805).
One of the managers appointed by the House of Representatives in 1798 to conduct the impeachment proceedings against William Blount, a Senator from Tennessee.