[1][2] He was born Unaarrimin, a member of the Jardwadjali people, on Mullagh Station, about sixteen kilometres north of Harrow, Victoria.

One player, "King Cole", died on the trip and, of those who returned, seven spent time on a reserve, two vanished, and the fate of two others is not clear from the surviving records.

[4] Mullagh's performances were impressive enough for him to join the Melbourne Cricket Club (MCC) as a professional for part of the 1869–70 season.

[7][8] Mullagh never appeared in an inter-colonial cricket match, although he did represent Victoria against a touring England side in 1879, top scoring with 36 in the second innings.

In February 2012, the Premier of Victoria, Ted Baillieu, and the Minister for Aboriginal Affairs, Jeanette Powell, announced that Mullagh would be one of the 20 inaugural inductees to the Victorian Indigenous Honour Roll.

In December 2019, Cricket Australia revealed plans for the Johnny Mullagh Medal, to be awarded to the best player in the Boxing Day Test match from 2020.