Name of U. S. Supreme Court Justice who had the longest tenure on the Court:

http://en.wikipedia.org/wiki/Us_supreme_court_justices

Scroll down to the chart and start doing some basic math, using the information in the Active Service column.

Examples --

James Wilson -- 9 years
John Jay -- 6 years
...

The U.S. Supreme Court Justice who had the longest tenure on the Court is William O. Douglas. He served as an Associate Justice from 1939 to 1975, for a total of 36 years.

To find this information, you can conduct a simple search on the internet using search engines like Google. You can simply type in the question "U.S. Supreme Court Justice with longest tenure" in the search bar, and the search results should provide you with the answer. This method usually leads to quick and accurate results for most factual questions.