Upon his return from Europe, McGavock worked as a lawyer in Nashville. McGavock was active in the
Tennessee Democratic Party. For example, he canvassed for
James Buchanan in the 1856 campaign. He served as Mayor of Nashville from 1858 to 1859. He had won the election thanks to the Irish vote. In 1860, he campaigned for
John C. Breckinridge. He was a strong proponent of states's rights. Prior to the
American Civil War of 1861–1865, McGavock established a militia in Tennessee among the Irish. Meanwhile, his wife founded the Ladies Soldiers' Friend Society, a patriotic group that included
Sarah Childress Polk, the widow of President
James Polk (1795–1849). During the war, he organized, outfitted, and served as lieutenant colonel of the 10th Tennessee Infantry ("Sons of Erin") in the
Confederate States Army. In 1862, he succeeded to command of the 10th Tennessee at
Fort Donelson when its colonel,
Adolphus Heiman, was given command of a brigade. He was captured in the ensuing siege and imprisoned in
Fort Warren on
Georges Island in
Massachusetts for five months. He was paroled in September 1862 and re-elected lieutenant colonel when the regiment was exchanged and reorganized. Upon Heiman's death from illness in November, McGavock became colonel of the 10th Tennessee and was killed in action leading a counter-attack at the
Battle of Raymond in Mississippi on May 12, 1863. ==Personal life==