Rotary mowers are often powered by Internal combustion engines. Such engines can be either two-stroke or four-stroke cycle engines, running on gasoline or other liquid fuels. Internal combustion engines used with lawn mowers normally have only one cylinder. Power generally ranges from two to seven horsepower. The engines are usually carbureted and require a manual pull crank to start them, although an electric start is becoming a sales feature in some countries.

Rotary mowers powered by are increasingly popular. Usually, these mowers are moved by manual motive power— the on-board engine or motor only spins the blades. These have the disadvantage of requiring a trailing power cord that limits its range and so these are only useful for relatively small lawns, close to a power socket. There is the obvious hazard with these machines of mowing over the power cable, which stops the mower and may put users at risk of electrocution. Installing a residual-current device (GFCI) on the outlet can reduce the risk of electrocution. Cordless (battery powered) electric lawn mowers are also available for small lawns. Electric rotary mowers weigh 45-50 pounds.

Rotary mowers typically have an opening in the side or rear of the housing where the cut grass is expelled. Some have a grass catcher attachment at the opening to bag the grass clippings. Special mulching blades are available for rotary mowers. The blade is designed to keep the clippings circulating underneath the mower until the clippings are chopped quite small. Other designs have twin blades to mulch the clippings to small pieces. This avoids the need for bagging the clippings or raking the clippings. Not only does this save labor, as no organics are removed from the lawn, less fertilizer is needed.

A dead man’s switch is required in some places so that the operator must hold a switch to keep the engine running. Typically, this is an extra bar that is held against the handle. Should the operator lose control of, or contact with, the lawn mower and release the bar, either the engine is turned off or the blade is disconnected by disengaging a clutch.