Peter signed on as an ordinary seaman in Dublin in 1716 when he was 13 years old. He rapidly rose in the ranks, becoming a Captain in 1727. His ship patrolled the American colonial waters to provide protection from French forces. He became involved in colonial politics and land speculation. In 1731 he married Susannah Delancey (1707-1771) whose brother James was the chief justice and lieutenant governor of New York.
Peter's lands included several thousand acres on the south side of the Mohawk River west of Schenectady, New York. In 1738 he hired his nephew William Johnson to manage western these lands. In 1745 Warren commanded a group of ships that supported the Massachusetts forces in the capture of Louisbourg. The prize system of the time allowed naval officers to profit from the capture of enemy ships, and this expedition earned the admiral a fortune. Peter and Susannah had six children, but two died in 1744 during the small pox epidemic in New York. He moved his wife and four surviving daughters to England in 1747. While on a visit to Ireland in 1752 he became ill and died.