Complex sentence: In 1990 he joined Pearn Kandola Occupational Psychologists in Oxford, where he was a partner from 1993 until March 2000 when he left to pursue his own interests.
Split into simpler sentences: In 1990 he joined Pearn Kandola Occupational Psychologists in Oxford. Having been a partner at Pearn Kandola from 1993 until March 2000, he left to pursue his own interests.