Michael Sadler, the youngest son of James Sadler, was born in Snelston, Derbyshire on 3rd January 1780. His family, though members of the Church of England, were in sympathy with the Methodist movement. At the age of seventeen Sadler published a pamphlet, An Apology for the Methodists (1797).

In 1800 Sadler moved to to Leeds where he worked for his father. In 1810 Sadler joined with his brother, Benjamin Sadler, to establish a company importing Irish linen. He also served as honorary treasurer of the poor rates. During this period Sadler became concerned about the condition of children working in local factories.

He argued against those who supported an unregulated society. He suggested that in a "society in which persons enjoyed unequal measures of economic freedom, it was not true that the individual pursuit of self-interest would necessarily lead to collective well-being." He added that "individual effort needed to be restrained and guided by the conscience of the community acting through the organisation of the state."

In 1829 the Duke of Newcastle offered Sadler the seat of Newark in the House of Commons. Elected with a majority of 214 votes, Sadler soon established himself as one of the best parliamentary speakers of the day. In 1831 Sadler moved to the safer seat of Aldborough.

Pamphlets published by Sadler included The State and Prospects of the Country (1829), The Factory Girl's Last Day (1830), On Poor Laws for Ireland (1830), On Ministerial Plan of Reform (1831) and On the Distress of the Agricultural Labourers (1831).

In October 1831 Sadler began his campaign to improve the condition of the agricultural labourers in Britain. He proposed: (1) the erection of suitable cottages by the parish authorities; (2) the provision of allotments large enough to feed a cow; (3) the provision of gardens to encourage horticulture among the labourers; (4) the provision of parish land for unemployed labourers.

Sadler was strongly supported by John Wood, who owned the most successful worsted spinning business in Britain. Wood donated £40,000 to Sadler's campaign for a ten-hour bill, aimed at reducing the excessive hours worked by children. According to his biographer, Gary Firth: "Throughout 1832 Wood canvassed for Sadler at Westminster, and that year he attended a large Easter rally in York.... In October 1832 Wood employed the Revd Matthew Balme as schoolmaster of a purpose-built school adjoining his mill."

On 16th March 1832 Sadler introduced legislation that proposed limiting the hours of all persons under the age of 18 to ten hours a day. He argued: "The parents rouse them in the morning and receive them tired and exhausted after the day has closed; they see them droop and sicken, and, in many cases, become cripples and die, before they reach their prime; and they do all this, because they must otherwise starve. It is a mockery to contend that these parents have a choice. They choose the lesser evil, and reluctantly resign their offspring to the captivity and pollution of the mill."

William James disagreed with Michael Sadler's views: "I have no doubt that the right honourable member (Michael Sadler) is actuated by the best intentions and motives, but I think that the course which he pursues will fail in attaining the object which he has in view. Undoubtedly the system which is pursued in these manufactories relating to the working of young children is a great evil; but it appears to me that the remedy which the honourable gentleman proposes to apply is worse than the disease. There appears to me to be only a choice of evils - the children must either work or starve. If the manufacturer is prevented working his mill for more than a certain number of hours together, he will often be unable to execute the orders which he may receive, and consequently, the purchaser must go to foreign countries for a supply. The result will be that you will drive the English capitalist to foreign countries, where there is no restrictions upon the employment of labour and capital."

Henry Thomas Hope agreed: "It is obvious, that if you limit the hours of labour, you will, to nearly the same extent, reduce the profits of the capital on which the labour is employed. Under these circumstances, the manufacturers must either raise the price of the manufactured article or diminish the wages of their workmen. If they raise the price of the article the foreigner gains an advantage. I am informed that the foreign cotton-manufacturers, and particularly the Americans, tread closely upon the heels of our manufacturers. The right honourable member (Michael Sadler) seems to consider that it is desirable for adults to replace children. I cannot concur with that opinion, because I think that the labour of children is a great resource to their parents and of great benefit to themselves. I therefore, on the these grounds, oppose this measure. In the first place I doubt whether parliament can protect children as effectively as their parents; secondly; because I am of the opinion that a case for parliamentary interference has not yet been made out; and thirdly, because I believe that the bill will be productive of great inconvenience, not only to persons who have embarked large capital in the cotton manufactures, but even to workmen and children themselves - that I feel it my duty to oppose this measure."

The vast majority of the House of Commons were opposed to Sadler's proposal. However, in April 1832 it was agreed that there should be another parliamentary enquiry into child labour. Sadler was made chairman and for the next three months a parliamentary committee, that included John Cam Hobhouse, Charles Poulett Thompson, Robert Peel, Lord Morpeth, and Thomas Fowell Buxton interviewed 89 witnesses.

On 9th July Michael Sadler discovered that at least six of these workers had been sacked for giving evidence to the parliamentary committee. Sadler announced that this victimisation meant that he could no longer ask factory workers to be interviewed. He now concentrated on interviewing doctors who had experience treating people who worked in textile factories.

Sadler was one of the chief speakers at the meeting organised by Richard Oastler at York on 24th April 1832. Later that year 16,000 people assembled in Fixby Park, near Huddersfield, to thank him for his work on behalf of child workers.

In the 1832 General Election, Sadler's opponent was John Marshall, the Leeds flax-spinning magnate. Marshall used his considerable influence to win the election and Sadler was now without a seat in the House of Commons.

Sadler's report was published in January 1833. The information in the report shocked the British public and Parliament came under increasing pressure to protect the children working in factories. Lord Ashley, son and heir of the 6th Earl of Shaftesbury, agreed to take over from Sadler as the leader of the factory reform movement in Parliament.

Michael Sadler died in Belfast on 29th July 1835.