His father served as a Wheelwright in the Royal Engineers but was wounded in action so became a storekeeper at the Woolwich Arsenal, London, where Henry was born. Henry began filling cartridges when aged 12. He then transferred to a woodworking shop followed by a smithy.
He got such a good reputation for his skill that Joseph Bramah (hydraulic press inventor) called for his services in making machines. Henry was good at putting other people's ideas into practice rather than an inventor himself. In 1791 married Bramah's housemaid Sarah Tindale. Six years later as Bramah's right-hand man and father of three, Henry asked for a raise in wages but was refused.
He then set up his own precision workshop (first, just off Oxford Street in central London, later - 1810 - in Lambeth). Following earlier work by Samuel Bentham, his first major commission was to build a series of 42 woodworking machines to produce wooden rigging blocks (each ship required thousands) for the Navy under Sir Marc Isambard Brunel. This was the first well known example of specialized machinery in an assembly line type factory.
He applied the ideas of interchangeable parts including nuts and bolts (before that all nuts and bolts were made as matching pairs only), and developed the first screw-cutting lathe, allowing standardisation on screw thread sizes for the first time.
Maudslay invented the first bench micrometer that was capable of measuring to one ten thousandth of an inch (he called it the "Lord Chancellor"), and a new form of slide rule. Having also developed a means to desalinate seawater for use in marine boilers, his company also specialised in marine steam engines.