goes looking

Apparently the last Democratic senator from Mississippi was John C. Stennis, but his entry into office predated the Civil Rights Act and the Southern Strategy and party realignment; he was a product of the Fifth Party System.

The second most recent was James Eastland, and it looks like the same sort of thing applies:

https://en.wikipedia.org/wiki/James_Eastland

James Oliver Eastland (November 28, 1904 – February 19, 1986) was an American attorney, plantation owner, and politician from Mississippi. A Democrat, he served in the United States Senate in 1941 and again from 1943 until his resignation in late 1978. Eastland was a segregationist who led the Southern resistance against racial integration during the civil rights movement, often speaking of African Americans as “an inferior race”.[1] Eastland has been called the “Voice of the White South” and the “Godfather of Mississippi Politics”.[2]

https://en.wikipedia.org/wiki/John_C._Stennis

Along with fellow Mississippi senator James Eastland, Stennis was a zealous supporter of racial segregation. He and Eastland supported the Dixiecrat ticket in 1948 headed by Strom Thurmond,[2] and signed the Southern Manifesto, which called for massive resistance to the Supreme Court ruling in Brown v. Board of Education. He also voted against the Civil Rights Act of 1964, the Voting Rights Act of 1965, and the Civil Rights Act of 1968.