Thomas Steers was born in 1672, probably at Deptford or Rotherhithe. He is thought to have had a good education, in view of his obvious skills in mathematics, and he joined the army during his teenage years. He was part of William of Orange's 4th Regiment of Foot (The King's Own), which fought at the Battle of the Boyne in 1690, and subsequently campaigned in the Low Countries against the French until the Peace of Namur was signed in 1697. He probably learnt about hydraulics at this time, a skill which served him well in later years. In 1698 or 1699 he married Henrietta Maria Barber, and her father gave them a house in Queen Street, Rotherhithe.
At the time, the Great Dock at Rotherhithe was being constructed, on land leased from Elizabeth Howland, which formed part of the Howland Estate. There is no record of Steers's direct involvement in the project, although he produced a survey of the completed docks in 1707, and seems to have been employed as a surveyor for the estate. A lease agreement at the time described him as a house-carpenter.
In 1708, plans for a dock at Liverpool, similar to that at Rotherhithe, were formulated, and had been drawn up by George Sorocold and Henry Huss by mid-1709. Neither man accepted the offer to act as engineer for the construction of the docks. On 17 May 1710, the Town Council learned that Steers was in Liverpool, and had his own designs for the project, which involved reclaiming land from the Pool, rather than building the dock of existing land. The precise reason for Steers' arrival in Liverpool is not clear, but may well be connected to the rise to power of James Stanley, who became mayor in 1707 and Lord Lieutenant of Lancashire until 1710, and who had noticed Steers in Flanders, while commanding the 16th Regiment of Foot. Steers' design was accepted, and the construction was overseen by him, assisted by William Braddock. He also contracted for some of the excavation work, and although it was incomplete at the time, the dock opened for shipping in 1715. A tidal basin and three graving docks or dry docks were authorised by another Act of Parliament obtained in 1717, and during their construction, various alterations and extensions were made to the original dock. The works were completed in 1721. Since 1717, Steers had also acted at Dock Master, for which he was paid £50 per year, and Braddock had been the Water Bailiff. From 1724, he took over Braddock's role as well, though was no longer paid, as this post included a number of perks and fees.
Concurrently with his work on the Liverpool Docks, Steers was active in other projects. He surveyed the rivers Irwell and Mersey from Bank Quay at Warrington to Manchester in 1712. An Act of Parliament authorizing the Mersey and Irwell Navigation was passed in 1721 and the work, which included eight locks in a distance of 15 miles (24 km) to overcome a rise of 52 feet (16 m), was completed about 1725. It is generally believed he was the engineer. The authorising Act named him as one of the Undertakers.
He also made surveys for the Douglas Navigation which connected the Ribble estuary to Wigan in 1712, and was again named as an Undertaker in the Act of Parliament obtained in 1720. He built a lock and a bridge, straightened a section of the river, and started the construction of a tidal lock, but his partner William Squire, who was raising finance for the scheme in London, became involved in the South Sea Bubble, and appears to have lost most of the money he raised. With the money gone, Steers moved on. The navigation was eventually completed in 1742, and carried coal from Wigan to Liverpool and onwards to Ireland by ship.