Hartwell received his bachelor's degree from the California Institute of Technology in 1961. In 1964, he received his PhD in biology from the Massachusetts Institute of Technology. From 1965 to 1968, he worked at the University of California as a professor. He moved to the University of Washington in 1968. In a series of experiments from 1970 to 1971, Hartwell discovered the cell division cycle (CDC) genes in baker's yeast (Saccharomyces cerevisiae). These genes regulate the cell cycle and mutations in the genes are involved in some types of cancer.
In addition to the Nobel Prize, Hartwell has received many awards and honors including the Louisa Gross Horwitz Prize from Columbia University in 1995. He became a member of the National Academy of Sciences in 1987. In 1996, Hartwell joined the faculty of Fred Hutchinson Cancer Research Center and in 1997 became its president and director.
In 1998 he received the Albert Lasker Award for Basic Medical Research. On July 9, 2003, Washington Governor Gary Locke awarded the Medal of Merit, the state’s highest honor, to Hartwell. He is also a recipient of the Komen Brinker Award for Scientific Distinction.