Greathead arrived in 1859 in England, where he studied with the noted civil engineer Peter W. Barlow (Barlow, Peter) between 1864 and 1867. The tunneling shield invented by Marc Isambard Brunel (Brunel, Sir Marc Isambard) and used to build the Thames Tunnel was large and unwieldy. Barlow designed a smaller shield, circular in cross section, which Greathead modified to complete the Tower Subway (1869) under the River Thames near the Tower of London. As the shield was pushed forward by screw jacks, the tunnel behind it was lined with cast-iron rings.

In 1886 Greathead began work to carry the City and South London Railway under the Thames near London Bridge, using a larger version of his shield, with which he bored twin tunnels about 10 feet (3 metres) in diameter. In this project he pioneered the use of compressed air (pneumatic device) in conjunction with the circular shield. His shield, compressed air, and the cast-iron rings used to line the tunnels came to be adopted generally in tunnel construction.