In 1801 the assistance of party friends enabled him to buy the Leeds Mercury. Provincial newspapers did not at that time possess much influence; it was no part of the editor?s duty to supply what are now called "leading articles", and the system of reporting was described as "defective" by the 1911 Encyclopedia Britannica. Baines made a complete change in the Mercury. His political articles gradually made the paper the organ of Liberal opinion in Leeds, and the connection of the Baines family with the paper made their influence powerful for many years in this direction. Baines soon began to take a prominent part in politics; he was an ardent advocate of parliamentary reform, and it was mainly by his influence that Macaulay was returned for Leeds in 1832; and in 1834 he succeeded Macaulay as member. He was re-elected in 1835 and 1837, but resigned in 1841.
In parliament he supported the Liberal party, but with independent views. Like his son Edward after him, he strongly advocated the separation of church and state, and opposed government interference in national education. His letters to Lord John Russell on the latter question (1846) had a powerful influence in determining the action of the government. He died in 1848. His best-known writings are:-The History, Directory and Gazetteer of the County of York; History, Directory and Gazetteer of the County of Lancaster; History of the County Palatine and Duchy of Lancaster. He was also the author of a History of the Wars of Napoleon, which was continued under the title of A History of the Reign of George III.
His Life (1861) has been written by his son, Sir Edward Baines (1800-1890), who was editor and afterwards proprietor of the Leeds Mercury, M.P. for Leeds (1859-1874), and was knighted in 1880; his History of the Cotton Manufacture (1835) was long a standard authority. An elder son, Matthew Talbot Baines (1799-1860), went to the bar, and became recorder of Hull (1837). He became M.P. for Hull in 1847, and in 1849 president of the Poor Law Board. In 1852 he was returned for Leeds, and again became president of the Poor Law Board (till 1855). In 1856 he entered the cabinet as chancellor of the duchy of Lancaster.