From the name of a county in Northern Ireland and a city in Pennsylvania. Its use as a given name seems to be due to the influence of the two film actors (father and son) called Tyrone Power, especially the younger one (1913–58).
English and Scottish: patronymic from the personal name John. As an American family name, Johnson has absorbed patronymics and many other derivatives of this name in continental European languages. (For forms, see Hanks and Hodges 1988.)