Born in Edinburgh and trained at the Glasgow Theological Academy, Stronach with his brother Alexander Stronach and their wives arrived in Singapore in March 1838. He was a secretary of the Singapore Tract and Book Society, assisted in English services, worked on a Malay New Testament, and encouraged a Malay girls' school run by his wife, Margaret. With Alexander he attended the August 1843 Hong Kong LMS Conference of Missionaries and the Convention of Missionaries on Bible translation.
From 1844 he and Margaret pioneered LMS work in Amoy (Xiamen), but she died en route to England in 1846. From 1847 to 1850, based in Shanghai, he was involved with the Delegates' Translation of the New Testament in classical Chinese; then, following disputes over the word to be used for God and other issues, he worked with other LMS missionaries on their own version. In 1853 he returned to Amoy, where he was based until 1876. He retired in 1878 and died in Philadelphia, Pennsylvania.