Lassiter attended McCabe's University School at Petersburg and was graduated from the law department of the University of Virginia at Charlottesville in 1886.[1] He was admitted to the bar in 1887 and commenced practice in Boston, Massachusetts. He returned to Petersburg, Virginia, in 1888 and continued the practice of law. He became the City attorney of Petersburg from 1888 to 1893. Lassiter was appointed by President Grover Cleveland to be United States attorney for the eastern district of Virginia in 1893 and served until 1896, when he resigned. He served as captain of Company G, Fourth Regiment, Virginia State Militia. He was appointed supervisor of the Twelfth Census for the Fourth Congressional District of Virginia in 1899.
Lassiter was elected as a Democrat to the Fifty-sixth Congress to fill the vacancy caused by the death of Sydney P. Epes. He was reelected to the Fifty-seventh Congress and served from April 19, 1900, to March 3, 1903.
1900; Lassiter was elected to the U.S. House of Representatives with 98.65% of the vote in a special election, defeating Independent James Seldon Cowdon; he was re-elected with 60.06% of the vote in the general election, defeating Republican C.E. Wilson.