Newcomen's engine consisted of a boiler A, in which the steam was generated. This was usually a haystack boiler, situated directly below the cylinder. It produced low pressure steam, all that the current state of boiler technology could cope with. Steam at this pressure would be unable to move a piston of any size.

One side of the beam was attached by a chain to the pump at the base of the mine, and the chain at the other side suspended a piston within a cylinder B. The cylinder was open at the top end, above the piston P, to the atmosphere. The piston had a bevelled edge, around which hemp rope, kept in place by metal weights, acted as a primitive seal. (The rope was kept wet, so that it would expand against the sides.)

When the valve V was opened, the steam was admitted into the cylinder. After this valve was closed, valve V' was opened to allow cold water from the tank C into the cylinder, thus condensing the steam and reducing the pressure under the piston. The atmospheric pressure above then pushed the piston down in the power stroke.

This raised the working parts of the pump, but their weight immediately returned the beam to its original position. Steam was then readmitted, driving the remains of the condensate out through a one way snifter valve as the process started all over again.

Early versions used manual operation of the valves to work, but the action was slow enough that this was not a serious concern. Later versions used controls attached to the rocking beam to open and close the valves automatically when the beam reached certain positions. The common story is that in 1713 a boy named Humphrey Potter, whose duty it was to open and shut the valves of an engine he attended, made the engine self-acting by causing the beam itself to open and close the valves by suitable cords and catches (known as the "potter cord"[1]). This device was simplified by 1718 according to an illustration by Henry Beighton, who showed suspended from the beam a rod called the plugtree, which worked the valves by means of tappets.

By 1725 the engine was in common use in collieries, and it held its place without material change for about three-quarters of a century. Towards the close of its career the atmospheric engine was much improved in its mechanical details by John Smeaton, who built many large engines of this type about the year 1770.

While its main use was pumping water out of mines, the Newcomen engine was also used in some places to pump water to drive machinery, for example refilling the upper pool at Coalbrookdale so that there was more water available to drive the blast furnaces, also at Madeley Wood or Bedlam Furnaces and others of the same period (the 1750s. Richard Arkwright, for example, even attempted to use a Newcomen engine to pump water to power a waterwheel.

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