Son of Harman Warner of London, merchant tailor, he was baptised at St. Clement Danes in the Strand on 17 September 1581.

When that parliament was dissolved, and the convocation continued its sittings under royal licence, Warner assisted William Laud in framing new canons.

On 4 August following he was impeached with other bishops by the House of Commons, under the stature of praemunire, for taking part in the convocation of 1640 and making new canons.

On 4 February 1649, within a week of the execution of Charles I, he preached and afterwards published anonymously a sermon alluding to it on Luke xviii.

[4] In 1649, on payment of a fine, the sequestrations on his property were discharged; but he refused to take the oaths to the usurping government, as he considered it to be.

In 1661 parliament recalled the bishops to the House of Lords, and once more, on 11 February 1662, Warner was able to address his clergy in Rochester Cathedral.

He died on 14 October 1666, aged 86, and was buried in Merton's Chapel in Rochester Cathedral, where a monument by Joshua Marshall exists to his memory.

[6] Gilbert Burnet had a copy of the Articles of the Barons, the heads of agreement for the Magna Carta, and he said it had come via John Lee from Warner, who had taken it from Lambeth Palace at the time of Laud's arrest on 18 December 1640.

The authorship of an anonymous manuscript diary giving eyewitness details of the House of Lords during the start of the Long parliament is attributed to him, for example by Conrad Russell.