Likens got his B.S. at Manchester University (North Manchester, Indiana) in 1957, and his M.S. (1959) and Ph.D. (1962) at the University of Wisconsin–Madison. He joined the faculty of Dartmouth College in 1961. He was co-founder in 1963 of a group with F. Herbert Bormann, Robert S. Pierce and Noye M. Johnson working on the Hubbard Brook Ecosystem Study at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. The study immediately found that the rain was abnormally acidic, and the group made one of the first scientific studies linking acid rain to air pollution such as sulphur dioxide. The group also devised a range of highly influential long-term experiments on the ecosystem scale.
In 1969, Likens joined the faculty of Cornell University. In January 1983 he was named Cornell University’s Charles A. Alexander Professor of Biological Sciences. While at Cornell University, he served as Chairman of the Section of Ecology and Systematics.
Likens was elected to the United States National Academy of Sciences in 1981. In 1983 he founded the (now Cary) Institute of Ecosystem Studies in Millbrook, New York as part of the New York Botanical Garden. The IES became independent in 1993 with Likens as director and president. In 2007 Likens stepped down as director of the IES and returned to full-time research, currently at the University of Connecticut and as Visiting Professor at the University of Uppsala, Sweden. In July 2012 he began a three year term as Special Advisor to the President on Environmental Affairs and Distinguished Research Professor.
Likens was also elected as foreign member of the Royal Swedish Academy of Sciences in 1988, the Royal Danish Academy of Sciences and Letters in 1994 and the Austrian Academy of Sciences in 2000. He was awarded the U.S. National Medal of Science in 2001.