Born to an English-speaking family, he learnt Welsh from his uncle.

Both Islwyn's elder brothers were engineers, and he was intended for the same profession training initial as a mining surveyor. However, he showed an aptitude for the ministry, and was sent to an academy run by Dr Evan Davies at Swansea. He was at one time tutored by a namesake, the poet William Thomas .

He became engaged to a local girl, Ann Bowen. Her death in 1853, at the age of twenty, became a source of poetic inspiration to him, and he was a regular winner of local Eisteddfod prizes from the 1850s onwards, taking his bardic name from the mountain Mynyddislwyn, above his home.

His two best-known poems are both entitled "Yr Ystorm" ("The Storm"), a long philosophic poem over 9,000 lines long. His poems are noted for their confident expressions of Christian faith, expectation of reunion in heaven, fulfilment of Christian duty and completion of a life fulfilled in God's work. He began preaching in 1854, and was ordained a Calvinistic Methodist minister in 1859 but never took charge of a church.

In 1864 he married Martha, Ann Bowen's stepsister. He edited several periodicals, and the Welsh column of the Cardiff Times. His poetry, although not always greatly regarded in his own lifetime found favour after his death and is now thought to be amongst the finest of the nineteenth century.

He also wrote twenty-nine poems in English.

He died from bronchitis in Ynysddu in 1878. He is buried in Babel Chapel, located in Cwmfelinfach.