Reared on farm; educated at home. (Doctor of Laws, Washington and Lee University). Apprenticed to tanner’s trade, 1843-1846.
McClure became interested in the newly formed Republican Party. He was an outspoken abolitionist. In 1857, he was elected to Pennsylvania's House of Representatives and re-elected in 1858 and 1859. He was elected to the Pennsylvania Senate in 1860.
At the 1860 Republican National Convention he became a well-known political figure, opposing fellow Pennsylvanian Simon Cameron's bid for the Republican nomination for the presidency. McClure and Andrew G. Curtin helped swing the state's vote away from Cameron and William Seward to Abraham Lincoln. After Lincoln's election McClure became chairman of the Republican state committee and helped to elect Curtin governor of Pennsylvania.
McClure was commissioned as an assistant adjutant general by Lincoln.
McClure's home in Chambersburg was threatened several times by Confederate forces. He was captured but released when Stuart entered Chambersburg on his raid around McClellan in October 1862. He states in his biography that he "never saw General Lee during the war or after the war." In 1864, during the Confederacy's third occupation of Chambersburg, McClure's home, Norland was burned down with much of the rest of the town. The home was rebuilt and sold to Wilson College.
In 1864, McClure moved to Philadelphia and helped Lincoln carry Pennsylvania again in the general election.
In 1867, McClure wrote a book called Three Thousand Miles through the Rocky Mountains. He also became a representative of the Philadelphia-based Montana Gold and Silver Mining Company and was superintendent of one of the company's mills at the Oro Cache vein in the Montana Territory. He returned to Philadelphia in 1868 after supporting Ulysses S. Grant at the Republican National Convention.
By the time of Grant's reelection bid, McClure had left the Republican Party and threw his support to Horace Greeley and the Liberal Republican National Convention. In 1873 McClure was elected to the Pennsylvania Senate. In 1874, he ran for mayor of Philadelphia and lost by only 900 votes.
He returned to newspaper editing by founding The Philadelphia Times in 1875. In 1886 McClure wrote The South: Its Industrial, Financial, and Political Condition, which included material on race relations in the South. McClure recognized that integration was necessary. He continued as The Philadelphia Times' editor until 1901 when he sold the newspaper to Adolph Ochs. In 1879 he married Cora M. Gratz.
Member of convention that organized Republican party, Pittsburgh, 1855. Member legislature, 1857-1858 and 1864.
Married Matilda S. Gray, February 10, 1852. Married second, Cora M. Gratz, March 19, 1879.