John Mortensen enjoys an unusually broad career as a performer and teacher of classical and jazz piano. His concerts may start with music of Bach, venture through several styles of music, and end with improvisation. He has performed with diverse artists such as Moscow Conservatory professor Mikhail Petukhov and All-Ireland fiddle champion Winifred Horan.

His students learn a natural and coordinated approach to piano technique, which prevents injury and allows for unprecedented freedom and facility at the keyboard. He also performs and teaches Irish and American roots music, playing mandolin, octave mandolin, Irish flute, Irish button accordion, five-string banjo, Uilleann pipes, and Irish whistle. He created America's only college-level traditional Irish music session class.

After his concert in Eisk, Russian Federation, the Russian press wrote that "... for John Mortensen Russia has always been close musically. He plays with especial passion the works of Sergei Rachmaninoff. ‘I don’t speak Russian, I speak Rachmaninoff,’ was heard from the mouth of the pianist during the concert. And truly, during the performance of the work of the great Russian composer, in the hall peoples of different nationalities disappeared — it seemed from the stage sang and wept the Russian soul. The chords of the next Rachmaninoff prelude had not even been played, and the hall was already conquered.”