His articles describing the massacre of Bulgarian civilians by Turkish soldiers and irregular volunteers in 1876 created public outrage in Europe, and were a major factor in preventing Britain from supporting Turkey in the Russo-Turkish War of 1877–78, which led to Bulgaria gaining independence from the Ottoman Empire.

There he met his cousin,[5] General Philip Sheridan, a Civil War hero also of Irish parentage, who convinced him to study law in Europe.

MacGahan's vivid articles from the front lines describing the stunning defeat of the French Army won him a large following, and many of his dispatches to the Herald were reprinted by European newspapers.

In 1875, he voyaged with British explorer Sir Allen Young on his steam yacht HMS Pandora on an expedition to try to find the Northwest Passage from the Atlantic to the Pacific.

[8] In 1876 MacGahan quarreled with James Gordon Bennett Jr., the publisher of the New York Herald, and left the newspaper[citation needed].

He was invited by his friend, Eugene Schuyler, the American Consul-General in Constantinople, to investigate reports of large-scale atrocities committed by the Turkish Army following the failure of an attempted uprising by Bulgarian nationals in April 1876.

[10] After visiting Philippopolis on July 28, and Peshtera and Pazardjik on August 1 and 2, MacGahan travelled to the village of Batak, and sent the paper a graphic report of what he saw:

An immense number of bodies had been partially burnt there and the charred and blackened remains seemed to fill it half way up to the low dark arches and make them lower and darker still, were lying in a state of putrefaction too frightful to look upon.

Here they show us a house where twenty people were burned alive; there another where a dozen girls had taken refuge, and been slaughtered to the last one, as their bones amply testified.

The government of Prime Minister Benjamin Disraeli, a supporter of Turkey, tried to minimize the massacres and said that the Bulgarians were equally to blame, but his arguments were refuted by the newspaper accounts of MacGahan.

Following the publication of MacGahan's articles, William Ewart Gladstone wrote a pamphlet called Bulgarian Horrors: "I entreat my countrymen," he wrote, "upon whom far more than upon any other people in Europe it depends, to require and to insist that our government, which has been working in one direction, shall work in the other, and shall apply all its vigour to concur with the states of Europe in obtaining the extinction of the Turkish executive power in Bulgaria.

[15][16] The 1877 invasion prompted this sardonic comment from MacGahan: “I can safely say I have done more to smash up the Turkish empire than anybody else ... except the Turks themselves.” He was in Constantinople, preparing to travel to Berlin for the conference that determined the final borders of Bulgaria, when he caught typhoid fever.