[9] Machen represented Kentucky's 1st congressional district in the First Confederate Congress, serving on the Accounts and Ways and Means Committees.

[2] After the close of the war, Machen, fearing reprisals for his alignment with the Confederacy, fled to Canada; his third wife and daughters Minnie and Marjorie joined him there.

[11] On July 9, 1872, Kentucky's delegates to the Democratic National Convention in Baltimore, Maryland nominated Machen for the office of Vice-President of the United States; he received one electoral vote.

[4] On September 22, 1872, Governor Preston H. Leslie appointed Machen to the United States Senate to fill the vacancy caused by the death of Garrett Davis.

[4] When the Kentucky Senate re-convened, he was formally elected to the seat on January 21, 1873, defeating Republican Tarvin Baker by a vote of 104–18.

[4] Following his term on the railroad commission, Machen retired to Mineral Mound, his 1,000-acre (4 km2) estate on the Cumberland River near Eddyville, where he raised tobacco.

[11] He died on September 29, 1893, at the Western Asylum in Hopkinsville, Kentucky, and was interred in Riverview Cemetery in Eddyville.