Kramer was born in Brookville, Indiana, and graduated in botany from the University of Miami, obtaining his PhD from Ohio State University in 1931. He immediately joined the faculty of Duke University, South Carolina, and spent his entire career there serving as professor of botany from 1945 until his retirement in 1974.

Kramer worked on problems of the absorption of water by plants, surveying the subject in his Plant and Soil Water Relationships (1949). He demonstrated that two different mechanisms are involved in water uptake by roots, depending on whether the plants are transpiring quickly or slowly. He also showed the importance of taking plant water stress into account when making correlations between soil moisture and plant growth. In studies using radioactively labeled elements he found that the region of maximum absorption in roots is not the tip but the area several centimeters behind the tip where the xylem conducting vessels are fully formed. Other researches led Kramer to the conclusion that substantial amounts of minerals enter plant roots passively in the transpiration stream.

Kramer also worked on the physiology of trees, publishing with Theodore Kozlowski The Physiology of Woody Plants (1979), an update of an earlier 1960 joint work.