He was born in Islington (then in the county of Middlesex, now in Greater London) on 8 December 1820,[3] the eldest son of John Mackarness, a West India merchant (died 2 January 1870), and Catherine, daughter of George Smith Coxhead, a physician.

By the recommendation of William Ewart Gladstone, he was appointed to the see of Oxford, being consecrated bishop on 25 January 1870, and invested as Chancellor of the Order of the Garter on 5 February 1870.

As a bishop, Mackarness was fearless and independent, without any trace of affectation, and the sermon which Ince (a professor) preached at Christ Church Cathedral, Oxford, on 22 September 1889, and afterwards published, bore public witness to the regard which the clergy of his diocese had for him.

When an attempt was made to force him to take proceedings against the rector of Clewer, he argued the case in person before the judges of the queen's bench division.

A liberal in politics, he voted in the lords against the Afghan war and the Public Worship Regulation Act, while he supported the bill for allowing dissenters to be buried in churchyards with services from their own ministers, and the measure for the removal of religious tests in the universities.

On surrendering to the ecclesiastical commissioners the management of the Oxford bishopric estates, Mackarness, with singular honesty, paid to them the sum of £1,729, being the estimated amount which he had received therefrom in excess of his statutory income during the previous nine years.