He completed his arts and theological studies, and offered himself to, and was accepted by, the London Missionary Society for the mission field in China.
In 1863 Williamson returned to China with the National Bible Society of Scotland as its first agent there.
He started at Yantai in Shandong Province and then travelled extensively distributing copies of the Bible in Chinese.
There at “Korea Gate,” Williamson sold Christian books to Korean border merchants.
In 1871, Williamson was awarded a Doctorate of Law by the University of Glasgow for his writings about China.