Dennis was born at his family home, Beverly, in Pocomoke City in the Province of Maryland, on December 17, 1771.

He completed preparatory studies at Washington Academy, attended Yale College; studied law; was admitted to the bar in 1793 and commenced practice in Somerset County, Maryland.

He later served two terms in the Maryland House of Delegates, before being elected as a Federalist to the United States House of Representatives, where he served from March 4, 1797 to March 3, 1805.

[1] During that time, Dennis was one of the House managers appointed in 1798 to conduct the impeachment proceedings against Senator William Blount of Tennessee.

[1] Dennis died in Philadelphia, Pennsylvania, where he was interred in Christ Church Burial Ground.