Born on 5 May 1850 at Cramlngton, Northumberland, he was largely self-taught. The son of a coal miner, he went down the pit at the age of 10 and at 17 became a hewer. By the 1870s he had become active in the Northumberland Miners' Union and went as a representative to the meeting of the TUC in 1884. In 1885, while still working in the pit, he became an MP, representing the new Wansbeck constituency as a Gladstonian Liberal. From 1890 to 1894 he was secretary of the TUC parliamentary committee, despite his known opposition to the eight-hour day for miners. He served on many royal commissions concerned with labour questions and in 1911 was made privy councillor in the coronation honours. In the 1911 election both he and his friend Thomas Burt stood as independent Liberals.
Fenwick was a PM local preacher for many years and was seen as an encouraging example of working-class achievement. He died at Newcastle upon Tyne on 20 April 1918.