Thomas Bayard was born in Wilmington, Delaware on October 29, 1828, the son of future Delaware senator James Asheton Bayard Jr. Although he did not attend college, Thomas Bayard studied law privately and was admitted to the Delaware bar in 1851.

After an extensive time practicing law in Wilmington, Bayard was elected to the United States Senate as a Democrat (1869-1885) and served as a key leader of his party during Reconstruction. He received some consideration for the Democratic presidential nomination in 1876, 1880, and 1884, and he was a member of the Electoral Commission that decided the contested presidential election of 1876.

After Grover Cleveland's election as President in 1884, Bayard became secretary of state (1885-1889). When Cleveland failed in his reelection bid, Bayard returned to Wilmington in 1889 and resumed his legal practice. Cleveland named him ambassador to Great Britain upon his return to the White House in 1893.

Bayard returned to the United States in 1897 with failing health and retired from public life. He died the following year at his daughter's home in Dedham, Massachusetts.