Denny was born on June 10, 1899, in Nashville, Tennessee. He was the son of Lucy Chase Chapman Denny and Collins Denny (1854–1943), who in 1910 moved the family to Richmond. Denny entered Princeton University, where he served in the Reserve Officer Training Corps, and in July 1918 joined the U.S. Army. He was commissioned a second lieutenant but did not serve in Europe during the final months of World War I (1914–1918). Denny returned to Princeton and graduated in 1921. After receiving a law degree from the University of Virginia in 1924, he established a Richmond law practice that became Denny, Valentine, and Davenport. Denny married Rebecca Smith Miller on September 10, 1932, at Brandy Station, in Culpeper County. They had two sons.
With his father, an influential bishop of the Methodist Episcopal Church South, Collins Denny Jr. opposed the reunification of the northern and southern branches of the Methodist Church, in large part because he feared that it would endanger the racial segregation practiced in the South. He supplied evidence in the 1930 church trial of Bishop James Cannon, an advocate of reunification, and with his father wrote An Appeal to Men of Reason and Religion Concerning Methodist Unification (1937), a pamphlet warning of the consequences of reunion. Denny was also an attorney for a group of South Carolina Methodists who unsuccessfully argued in federal courts for the right to retain the name Methodist Episcopal Church South after the denomination unified in 1939.
In Virginia, Denny aligned himself with the dominant faction of the Democratic Party headed by Harry F. Byrd Sr. and was an assistant attorney general of Virginia from 1930 to 1934. He was appointed to the state textbook commission in 1948 to inquire into the materials and methods used to teach Virginia history and government in the public schools. That same year Denny, in collaboration with Governor William M. Tuck, drafted a resolution adopted by the Democratic Party of Virginia that was critical of the civil rights policies of President Harry S. Truman. After delegates from several southern states left the Democratic National Convention to protest its civil rights platform, Denny attended as an observer the formation of the States Rights Democratic Party (popularly known as the Dixiecrats) in Birmingham, Alabama. He publicly supported the party’s presidential candidate, South Carolina governor J. Strom Thurmond, and made several speeches for Thurmond in Virginia. Denny never again voted for a Democratic presidential candidate.
In 1949 as counsel for the Atlantic Coast Line Railroad, Denny unsuccessfully defended the railroad’s practice of segregation in interstate transportation. He interpreted the Supreme Court’s 1954 ruling in Brown v. Board of Education, that mandatory racial segregation of public schools was unconstitutional, as the ultimate attack on southern society. At a well-attended public meeting in South Boston, he denounced the court’s opinion as “a dastardly decision, seeking to beat down our way of life and to regiment us.” That year Denny assisted in founding the Defenders of State Sovereignty and Individual Liberties, dedicated to preserving racial segregation. He was its legal counsel, drafted some of its public statements, and helped develop its plan to use every legal and political means to block school desegregation. Central to Denny’s strategy was the substitution of segregated private schools subsidized with state funds for any public school facing federal desegregation orders. In September 1956 he met with Governor Thomas B. Stanley and his chief advisers to formulate the legal details of the state’s policy of Massive Resistance to court-ordered desegregation. In testimony before the General Assembly that same month Denny supported the governor’s plan to close and withhold state funds from any public school ordered to admit African American students.
Two years later Governor J. Lindsay Almond Jr. closed public schools in three localities but then reversed himself and backed down. Denny was disgusted. Prince Edward County, where the Virginia suit that was incorporated into Brown v. Board of Education had arisen, closed its public schools in June 1959 rather than desegregate them. The county school board hired Denny to represent it when black parents represented by the NAACP challenged the county’s failure to appropriate funds for public education. During the next four years, he defended the school board as the case made its way through state and federal appellate courts. An able constitutional lawyer and astute reader of the Supreme Court’s opinions, Denny predicted in 1962 that the court would soon require even the county’s private academies to admit students regardless of color.
Denny also represented school boards in Powhatan and Surry counties, where segregation was challenged, but there was a paternalistic element present in his racial views. On his deathbed he desired that memorial contributions be given to the white Virginia Education Fund and also to the Prince Edward Free School Association, which provided education for some of the county’s black students. After suffering several years of poor health, Denny died at his Powhatan County home on January 14, 1964, and was buried in Richmond’s Hollywood Cemetery, before the Supreme Court heard arguments in the Prince Edward case. As he had predicted, in May of that year the court ruled against the county and ordered the public schools reopened.