Origin

In 1939 during the New York City World Fair Montague Dexter and his wife went off in a space ship only to crash on mars and live there for decades.  It was on this planet that Dex was born and by the year 2000 he had helped his father fix there ship and Dex left his father and went back to earth only to crash again into another space ship that was headed to earth. 
 
Fortunately for Rex he survived the crash and was taken back to New york City by the very ship he crashed into.  There he stops Boris Thorax from destroying America.

Creation

Rex Dexter was created by Dick Briefer.  There are some mix ups in Rex's origin In Rex's first appearance he was the son of Montague Dexter and the first man to be born on Mars.  In the second issue he is the grandson.  The future seems to jump with each issue as well.  
 

Special Notes

Rex Dexter's phone number is R,D,21,M.  Thats R-D-21-M.