After the Confederate defeat and his release from prison after the war, Reagan called for cooperation by the southern states with the U.S. government, an unpopular position among most conservative whites.

Alexander H. Stephens, the only Confederate vice president, was also elected in 1866 to represent Georgia, but was refused to be seated to the Senate due to his war history.

Elected as the representative of the Democratic Party for the state of Texas, he sat in the Senate for just one term; he was one of just three former Confederate cabinet members to take major political offices after the war.

He believed abolition would cause such social problems as to require Southern whites "exterminate the greater portion of the [black] race.

Chosen as a member of the Provisional Confederate Congress, President Jefferson Davis appointed Reagan to his cabinet as Postmaster General within a month.

Despite the hostilities, the United States Post Office Department continued operations in the Confederacy until June 1, 1861, when the Confederate service took over its functions.

[citation needed] Reagan cut expenses by eliminating costly and little-used routes and forcing railroads that carried the mail to reduce their rates.

He instead supported a proposal to detach the First Corps of the Army of Northern Virginia to reinforce Joseph E. Johnston in Mississippi, to break the Siege of Vicksburg.

Historian Shelby Foote noted that, as the only Cabinet member from west of the Mississippi, Reagan was acutely aware of the critical consequences of Vicksburg's capture and control of the river by U.S. forces.

When Davis abandoned Richmond, Virginia on April 2, 1865, shortly before the entry of Army of the Potomac under George G. Meade, Reagan accompanied the president on his flight to the Carolinas.

On August 11, he wrote an open letter to his fellow Texans urging cooperation with the United States, renunciation of the secession convention, the abolition of slavery, and letting formerly enslaved people vote.

Later that year, Reagan died of pneumonia at his home in Palestine, the last surviving member of Jefferson Davis' cabinet in the Confederate government.