Starting as lecturer at Rivers State University in food chemistry/biochemistry, food process technology, safety and fermentation, toxicology and nutrition in 1975, he has remained there to-date. He is also a researcher in Rivers State University, Port Harcourt (formerly Rivers State University of Science and Technology) and research is mostly centered on the composition and quality evaluation of Nigerian local foodstuff.
As of 2021, Achinewhu serves as professor emeritus at the Rivers State University.
Prior to being appointed vice-chancellor of Rivers State University 2000, Achinewhu held the following positions:
* Dean, Post Graduate School (1998-2000)
* Head of the Department of Food Science & Technology (November 1995 – June 1998)
* Dean, Faculty of Agriculture (1989-1994)
* Director, Rivers Institute of Agricultural Research & Training (RIART), Rivers State University (1986-1989)