John McClellan was born in Sheridan, Arkansas, on 25th February, 1896. He studied law and was admitted to the bar in 1913. During the First World War McClellan served in the United States Army and reached the rank of first lieutenant in the Signal Corps.

After the war McClellan established himself as a lawyer in Malvern, Arkansas. In 1927 he was appointed prosecuting attorney of the seventh judicial district of Arkansas.

A member of the Democratic Party, McClellan was elected to the Senate and took his seat in January, 1939. Over the next 40 years McClellan served on the Committee on Expenditures, Committee on Government Operations and the Committee on Appropriations.

McClellan was also chairman of the Permanent Investigations Committee. On 24th June, 1962, McClellan announced that his committee would be looking into the activities of Billie Sol Estes. On 27th July one witness testified that Lyndon B. Johnson was getting a rake-off from the federal agricultural subsidies that Estes had been obtaining.

Estes' trial began in October 1962. John Cofer, who was also Lyndon Johnson's lawyer, refused to put Estes on the witness stand. Estes was found guilty of fraud and sentenced to eight years in prison. Federal proceedings against Estes began in March 1963. He was eventually charged with fraud regarding mortgages of more that $24 million. Estes was found guilty and sentenced to fifteen years in prison.

The Permanent Investigations Committee continued to look into the case of Estes. President John F. Kennedy now began considering dropping Lyndon B. Johnson as his running-mate in the next presidential election. According to Barr McClellan it was now decided by Edward Clark that this investigation had to be brought to an end. McClellan claims that Clark recruited Mac Wallace to organize the assassination of Kennedy. When Johnson became president he managed to bring the Senate investigations into Billie Sol Estes and Bobby Baker to an end.

John McClellan died in Little Rock, Arkansas, on 28th November, 1977.