He studied in Copenhagen under Christoffer Wilhelm Eckersberg, in Dresden under Eduard Julius Bendemann, and in Paris under Thomas Couture. Afterward he lived much in Rome and in Paris, where he constantly exhibited at the salons. In 1877 he was appointed professor at the Royal Danish Academy of Art, Copenhagen. His illustrations, especially of children's books, are known everywhere and are more important than his paintings. His daughter Edma Frølich was his favorite model when a baby and a child for his French albums with Pierre-Jules Hetzel.
He also furnished original etchings for Adam Kristoffer Fabricius' Illustrated History of Denmark; for Apuleius' Cupid and Psyche; The Lord's Prayer; Die Götter des Nordens, and many other works. He painted a decoration in the Court of Appeals at Flensburg, Schleswig-Holstein, and in some public buildings of his native land.
Dagmar Olrik and her assistants spent 18 years decorating a room in Copenhagen's City Hall with tapestries based on cartoons of Nordic mythology by Frølich.
Carl Nielsen composed the Kantate til Lorenz Frølich-Festen (CNW 103) for Frølich's eightieth birthday, celebrated in Koncertpalæet, Copenhagen on 30 November 1900.