He worked as a schoolmaster in Banbury, Oxfordshire, until the outbreak of the English Civil War, at which point he became preacher to a London parish.
Barker was an avid parliamentarian and was invited to preach a sermon before the House of Commons on 25 October 1648.
The new republic welcomed him, and his moderation earned him the favour of the Cromwell regime, which made him an assistant to the London commission.
After the Restoration of the English monarchy, Barker became a nonconformist, forming his own London congregation.
To this end he published a collection of advice for scholars entering the ministry, entitled Flores intellectuales.