William Sylvis was born on November 26, 1828, in Armaugh, Pennsylvania. He was one of 12 children belonging to Nicholas and Maria Sylvis. Taught by his father, wagon making was William's first profession. He left home at age 18 with no formal education to start an apprenticeship at Forest Iron Works. After Forest Iron Works went out of business, he completed his iron training at a foundry in Curwensville, Pennsylvania. For a few years, Sylvis traveled throughout the state in search of a place to establish himself. During his travels, he met and married Amelia Thomas and eventually settled down in Philadelphia where he found work at the Cresson Foundry.

With the iron industry growing in the mid-1800s, skilled iron molders started hiring helpers in order to increase output at cheaper costs. Because foundries were hiring unskilled workers to help in production, those who were skilled in the iron industry saw their wages cut. In 1857, Sylvis joined the Journeymen Stove and Hollow-Ware Moulders' Union of Philadelphia when the union began a strike to demand better compensation. Within a few months of joining the union, Sylvis was elected the secretary. He began contacting the leaders of other iron molder unions in other cities in efforts of creating a national union. In 1859, delegates from 12 different iron molder unions met in Philadelphia to draft the constitution of the future national union. Sylvis was elected treasurer of the new organization, but when the Civil War started, the newly formed union was put on hold.

After serving in the Union army during the Civil war, Sylvis tried to reorganize efforts to create a national union. In an essay entitled "The Respectability of Labor," Sylvis wrote:

We look upon every kind of labor as respectable, because necessary; and no man, should he reach the most exalted position in life, could possibly lessen the dignity of himself, or compromise the sphere in which he moved, by resuming the humble occupation from which he sprung, for either pastime or convenience, so long as he faithfully discharged the duties of the position.

In 1863, Sylvis gathered 21 locals to form the Iron Moulders' International Union (IMIU) and was elected President. Sylvis began a campaigning tour in which he told molders not to allow political, racial, or religious differences to divide them. With his inspiring and motivating speeches, Sylvis rallied over 8,600 molders to join the IMIU. In 1866, Sylvis and other trade reformers met in Baltimore to form the National Labor Union (NLU), which aimed at reforming industrial order in the United States. He was elected president of the NLU in 1868 and rallied a total of 300,000 members before he died on July 27, 1869, due to severe inflammation of the bowels.