At a statewide convention of Tennessee Unionists in January 1865, Mullins was nominated for the seat representing Bedford and
Rutherford counties in the
Tennessee House of Representatives. He was elected to the seat on March 4 of that year. The legislature convened in April, and quickly ratified the
Thirteenth Amendment, which outlawed slavery. While the state legislature was dominated during this period by Southern Unionists (ex-Confederates were barred from voting), a rift quickly developed between the "conservatives," who were aligned nationally with President
Andrew Johnson, and the "radicals," who supported Governor
William G. Brownlow, and were aligned nationally with the
Radical Republicans. Conservatives, led by Speaker of the House
William Heiskell, generally sought a return to pre-Civil War conditions (though with slavery outlawed), while Radicals wanted to extend voting rights to African-Americans and punish former Confederates. Mullins voted with the Radical block, and became one of Brownlow's staunchest allies in the state house. Reports differ as to Mullins' response, with at least one stating he had to be restrained from attacking Heiskell, The franchise bill passed a few weeks later. The
Cleveland Banner, on the other hand, stated that Mullins was "making an ass of himself." In July 1866, Brownlow called a special session of the General Assembly to consider the
Fourteenth Amendment, which extended civil rights to African Americans. Conservatives vehemently opposed the amendment and organized another quorum bust. After two of the quorum-busting legislators were arrested and confined to the House chamber, the radicals argued this constituted a quorum, and pushed through the vote in favor ratifying the amendment. Heiskell resigned the Speakership in disgust. Mullins served as Speaker
pro tempore at various times in late 1866 and early 1867. ==Congress==