A native of Goochland County, near Richmond, Virginia, Warner T. McGuinn was born less than two years before the Civil War in November 1859. His parents, Jared and Fannie McGuinn, sent him to public school in Richmond and then he went on to graduate from Lincoln University in 1884. Warner McGuinn studied law at Howard University for two years but finished his degree at Yale, where he served as the president of the Law Club and made friends with Mark Twain before graduating in 1887. Twain even supported McGuinn's education after finding out that the young man was working his way through school.

McGuinn moved to Baltimore in 1890 and was admitted as a lawyer to the Maryland Bar in 1891. The next year he married Anna L. Wallace, a fellow Virginian, and started a family with the birth of their daughter Alma in September 1895. McGuinn started working with Harry S. Cummings, Baltimore's first African American City Councilman in 1893, and moved to 1911 Division Street, just six blocks north of Cummings' house on Druid Hill Avenue.

McGuinn participated in Civil Rights struggles and Republican politics throughout his life in Baltimore. In 1910, McGuinn and W. Ashbie Hawkins worked together to overturn the West segregation ordinance and McGuinn argued against a similar ordinance in court in 1917. In 1911, he voiced his support for women's suffrage by reading an "exhaustive" paper on the issue to an assembly gathered at Bethel A.M.E. Church to inaugurate the Baltimore Historical and Literary Association. The Afro-American Ledger reported that McGuinn reminded his audience of the principle of the consent of the governed found in the Declaration of Independence—making it evident that all adults had a right to participate in electing their own representatives regardless of their color or gender.

Warner T. McGuinn served two terms as a Republican on the Baltimore City Council, from 1919 to 1923 and 1927 to 1931. In May 1919, after his first election, the Afro-American quoted the new Councilman who said:

"I shall do my best in the City Council to fulfill every pledge that has been made during the campaign, especially as regards the health and school conditions of the race."

In 1927, the Sun praised his service as a Councilman, writing:

"No member has been more efficient or more earnest in endeavoring to promote public welfare than Warner T. McGuinn... He set an example of nonpartisanship in consideration of measures before the Council, and when he spoke upon them showed that he had taken pains to inform himself. His record deserves commendation."

While visiting his daughter Alma in Philadelphia, Warner McGuinn died on July 10, 1937. His home on Division Street still stands.