After the merger, Lackawanna continued to be a center for the manufacture of steel throughout most of the 20th century. In the 1940s, the Lackawanna steel mill employed over 20,000 people, and was the world’s largest steel factory.
In the 1970s and 1980s, Bethlehem Steel allowed the Lackawanna Steel plant to once again become obsolete. Foreign competition made it financially impossible to continue to manufacture most of the products produced at Lackawanna. Bethlehem Steel also disliked the high tax rates of the state of New York, and did not want to spend the millions of dollars in air and water pollution abatement which were required by state and federal authorities. The company built a new facility in Burns Harbor, Indiana, and stopped investing in new steel production methods at Lackawanna.
On June 25, 1982, Bethlehem Steel announced it would close the Lackawanna facility and lay off its remaining 10,000 workers in six weeks. The Lackawanna Steel Co. plant closed on October 15, 1982. The company laid off workers in waves before the final closure, and transferred many others. On the day the plant closed, more than 6,000 workers lost their jobs.
While the site has been declared a Superfund site, a Class 2 Inactive Hazardous Waste Site, and a brownfield the city of Lackawanna redeveloped some of the former Lackawanna Steel Co. land into small business zones, bringing about 700 jobs back to the town in the late 1980s. In 1993, Veritas Capital, Inc., purchased one of the bar, rod and wire plants on the old Lackawanna site and added another 250 jobs. However Bethlehem Steel ended coke production at the Lackawanna site in 2001, and as of 2008 only a galvanized steel finishing plant employing about 250 people remained.
Steel Winds, an eight-windmill wind farm owned by BQ Energy, opened on the old plant's slag heaps in September 2006. The rest of the site has been undergoing a slow demolition.