[2] Both of his parents were of Scottish ancestry,[3] and his father was a prominent businessman with Boorman, Johnston, & Co. and was a co-founder of Washington Square North.

His mother had four siblings who, likewise, married two grandchildren, a great-granddaughter, and a nephew of founding father Roger Sherman, Signer of the U.S. Constitution and the U.S.

He later studied at Yale Law School, where his classmates included Charles Astor Bristed, Daniel D. Lord, and Henry G.

He held this position until ill health forced him to retire in 1889, at which point he was succeeded by Henry Gurdon Marquand and the museum's Trustees voted him Honorary President for Life.

His personal art collection in his Fifth Avenue mansion, which included works by Frederic Edwin Church, Thomas Cole, Asher Brown Durand, John Frederick Kensett, and Winslow Homer,[5] In addition to his patronage of the arts, Johnston served as President of the Governing Board of the University of the City of New York, and as a member of the boards of the Presbyterian Hospital, the Woman's Hospital of New York, and the Saint Andrew's Society.

[12] Johnston was an active diarist, recording details of his travels through Europe and the United States as well as significant personal and world events, including his wedding excursion, trips with his family, a visit to Richmond, Virginia in 1865 after the surrender of the Confederate Army, and a trip west on the newly built Union Pacific Railroad.

Johston Drive in Watchung,the borough just north of his Plainfield Home at 857-859 East Front Street, was named after Johnston.