A firm Calvinist, he was a leader of mercenary troops in the religious wars of the time, including the Dutch Revolt.

In 1564 John Casimir suggested himself as a bridegroom for Elizabeth I of England and sent her his portrait via the Scottish courtier Sir James Melville.

His son, Louis VI, inherited the main part of the Palatinate, including Heidelberg, and John Casimir inherited a smaller portion, which became the independent Count Palatine of Lautern (essentially consisting of the city of Kaiserslautern and surrounding area).

John Casimir was in regular contact with Robert Dudley, 1st Earl of Leicester and his nephew Sir Philip Sidney who, as agent for Elizabeth I, was sent to the continent to assist in the formation of a Protestant league.

[4] He visited England in 1579 to seek the Queen's financial support for his campaigns on behalf of the United Provinces.