Clergyman. Welsh was born in Dunscore (Dumfriesshire), the son of the Laird of Collieston, and is noted as a wayward youth who played truant from school and then joined a gang of thieves. However he found God, sought forgiveness from his father and entered the newly-founded the University of Edinburgh, graduating in 1588. He became a minister in Selkirk, later moving to Kirkcudbright and then Ayr. He married Elizabeth, the youngest daughter of the father of the Scottish Reformation, John Knox (c.1513-72).
However he stood in the way of King James VI's plans to subvert Presbyterianism. Welsh preached a notable and defiant sermon in St. Giles Kirk in 1596 and continued to irk James VI over the following decade. After a brief trial in 1605, he was imprisoned first in the Tolbooth in Edinburgh and then Blackness Castle. Welsh was banished to France, sailing from Leith on the 7th November 1606 where he met like-minded individuals. He was allowed to return to London in 1622, where he died. He was buried in St. Botolph's Church on Bishopsgate. His grandson, another John Welsh (c.1624-81) continued to support the Presbyterian cause as a notable Covenanter.