Sinking restarted in earnest in 1921. Water flooded into the shafts as fast as the engineers could pump it out, until they solidified the surrounding ground with liquid cement grout, a technique that had only recently become practical at depth. On 29 October 1923 the shaft sinkers finally reached the Barnsley seam, 848 metres down. A second shaft hit the same seam on 15 November. Harworth was a deep mine by any standard, drawing coal from a depth where the rock was warm and the air had to be forced down through one shaft and back up the other to keep the workings safe. In 1924 the colliery was joined to the East Coast Main Line by a 4.2 kilometre branch, anchored by a 256 ft viaduct over the River Ryton.

Harworth coal had a reputation. The top layer of the Barnsley seam was excellent for steaming, the lower parts good for domestic and coking purposes, and the LNER bought heavily. There is a specific moment in the pit's history that gets pulled out by anyone who tells its story: in 1932, the Flying Scotsman, then the most famous locomotive in the world, ran the 392 miles from King's Cross to Edinburgh Waverley in seven hours and twenty-seven minutes - a record at the time. The fuel in the firebox was Harworth coal. By the late 1960s steam had given way to diesel and electric traction, gas was being made from feedstocks other than coal, and Harworth's market shifted decisively toward power generation. The big customers became coal-fired stations like Drax and Cottam, fed by trains running over the South Yorkshire Joint Railway.

The pit was modernised in the 1950s, with new concrete headgears that replaced the original Victorian-style winding towers. Coke ovens stood alongside. By the late 1980s, the headgears were replaced again, the No. 1 shaft in 1989 and No. 2 in 1994, and a new surface main fan was commissioned to ventilate the workings to modern standards. In 1993 Harworth hit a million tonnes in a year. But the wider British coal industry was contracting fast, and by November 2002 the owners UK Coal warned the 400-strong workforce that the pit could not absorb £8 million in annual losses indefinitely. The only viable plan was a £50 million investment to access a new seam, which would have given the pit another twenty-five years. The investment never came. In 2006 the mine was mothballed, ending eighty-six years of coal mining in Bassetlaw.

After the mothballing, UK Coal kept the option of reopening the pit on paper, even doing underground work in 2008 to make several kilometres of roadway passable again. By April 2016 the calculation had changed. The headgear, which had been the visible landmark of the town for generations, was demolished to make way for housing. In 2017, the Harworth Group announced plans to build 1,200 homes on the colliery site. The development was named Simpson Park, after Tom Simpson, the former world champion road cyclist who grew up in Harworth and who remains, with the colliery gone, the town's most internationally known native. A century-and-a-half cycle had run its course: agricultural land, then a coal village that drew workers from far away, then a pit that fed the railways and power stations of an industrial nation, then a redevelopment site for the houses of people who would commute somewhere else for work.