After that he was a post-doctoral fellow at the IBM T.J. Watson Research Center for a year before joining the faculty of the University of Toronto in 1980.
He has made fundamental contributions in the areas of graphical query languages, knowledge-base systems, and on-line analytic processing.
Together with his thesis advisor, Jeffrey Ullman, and fellow Princeton students, including David Maier and Yehoshua Sagiv, he co-authored a number of influential papers that laid out the fundamental issues and approaches for relational databases.
In a now-famous paper (Maier, Mendelzon and Sagiv, TODS 1979), he introduced the chase, a method for testing implication of data dependencies that is now of widespread use in the database theory literature.
In addition, he also made important contributions to recursive query languages, on-line analytic processing, similarity-based queries, data warehouses and view maintenance, algorithms for computing web page reputations, and indexing of XML data.