After being a monk for a short time he embraced the reformed religion and went to London, where, about 1542, he became chaplain to Henry Grey, afterwards Duke of Suffolk, the father of Lady Jane Grey.
On the accession of Mary to the English throne in 1553 he went to Emden in Friesland, where he practised as a physician, varying this profession with visits to Scotland. He was associated with the leading Scottish reformers in their opposition to the queen regent, Mary of Lorraine, and the Roman Catholic religion, and in 1558 he returned definitely to his native land.
Willock now began to preach and in 1559 was outlawed. Popular sympathy, however, rendered this sentence fruitless, and in the same year, being Knox's deputy as minister of St Giles' Cathedral, Edinburgh, he frustrated the efforts of the regent to restore the Roman Catholic religion, and administered the communion for the first time in accordance with the ideas of the reformers. He was one of the four ministers chosen by the convention of October 1559 to seats on the council of government, and was one of those appointed to compile the First Book of Discipline.
About 1562 he became rector of All Saints Church, Loughborough in Leicestershire, but he retained his connection with the Scottish church and was Moderator of the General Assembly of the Church of Scotland in 1562, and again in 1564, in 1565 and in 1568. He died at Loughborough on the 4th of December 1585.