Robert Lee Thornton Sr. (often R. L. Thornton; August 10, 1880 – February 15, 1964) was an American banker, civic leader, and four-term Mayor of Dallas, Texas.
A child of tenant farmers, Thornton's early years were divided between school and farm labor. After holding several jobs and starting two unsuccessful business ventures, Thornton began a banking operation in Dallas in 1916. The bank progressed to be a Texas-wide institution, and by 1923 it had a national charter. Thornton served as president (1916–1947) and board chairman (1947–1964) of the bank he founded, the Mercantile Bank and Trust Co.
Thornton became a prominent businessman and community figure, serving as president of the Dallas Chamber of Commerce from 1933 to 1936, and as president of the State Fair of Texas from 1945 to 1960. He played a major part in securing the Texas Centennial Exposition for Dallas. From 1953 to 1961, Thornton served as mayor of Dallas. His vigorous promotion of the city and its development earned him the soubriquet Mr. Dallas in the media. His avuncular and countryfied manner saw him often referred to and addressed as Uncle Bob by locals and associates.
Beginning in 1994, claims that Thornton had been a Ku Klux Klan member in the early 1920s have arisen. Records of any Klan association are scant and have been contested. His family argue that although Thornton was a highly scrutinized public figure, as mayor and prominent business leader, no allegations of Klan affiliation surfaced during his lifetime, nor for thirty years after his death. Thornton's involvement in the city's desegregation and civil rights matters in the 1950s and 60s provided an opening for criticism from African American and other civil rights campaigners; however there are no contemporary, nor later, accusations on record, prior to the one surfacing in 1994.
Thornton married in 1905 and settled in Dallas with his wife, where they raised four children. Thornton died in 1964. Various roads and places in Dallas are named for him. Since the 1990s there have been intermittent calls for these place names to be changed on the basis of Thornton's perceived involvement with the Klan. Although little firm evidence of such involvement appears to exist, the claim of his membership is repeated from time-to-time in various publications.