John, his wife Margaret and children Archibald and Mary Jane sailed to America in 1855 on the ship S. Curling. They had another son John that died before starting on the trail. Their surname is spelled "Johnson" on the ship S Curling and 1870 Utah census.
The Johnston family came to Idaho in either 1863 or 1864. They likely came in 1863, the year they took a PEF loan for their emigration and sailed to America. But the first documentation to place them in Idaho is the birth of a child in January 1865 which was prior to the arrival of that season's emigration. Further research is needed to narrow the year of their travel.