He experienced a dramatic conversion while John Wesley was preaching in Bristol in 1739 and soon became Wesley's lay assistant. In 1741 he was left in charge of the Foundery society while Wesley was in Bristol. Learning that Maxfield had begun to preach, Wesley was about to intervene, but his mother Susanna persuaded him to hear Maxfield first. After hearing him, Wesley acknowledged his call from God and appointed him one of the first of his lay preachers, referring to him as his first 'son in the gospel'. While preaching in Cornwall in 1745 he was press-ganged for the navy and then imprisoned and conscripted into the army; Wesley's attempt to intervene on his behalf was not immediately successful. He was later ordained by the Bishop of Derry to give Wesley support.
In the 1760s, supported by the visionary George Bell, he ran to fanatical extremes over Christian perfection and apocalyptic predictions. Both John and Charles Wesley tried, unsuccessfully, to reason with him, but he left Methodism in 1763 and became the leader of a breakaway group. He later opened a large chapel in Moorfields, London. Despite this, in letters to Charles Wesley John Fletcher wrote appreciatively of Maxfield's qualities, after the latter had preached for him in Madeley in 1765.
In 1767 Maxfield published a Vindication very critical of both the Wesleys, but John Wesley did not reply until Maxfield had returned to the attack in 1778. Before he died there was some healing of the rift and John Wesley preached in his chapel twice in 1783 and visited him in his final illness. He died in London on 18 March 1784.