Alexander Brook studied at the Art Students League in New York under Kenneth Hayes Miller. According to Raphael Soyer, "Kenneth Hayes Miller was the most influential teacher at the League. He was long-nosed and grim-visaged, but had a kind smile. He taught some of my brilliant contemporaries - Peggy Bacon, Reginald Marsh, Alex Brook, Kuniyoshi, Isabel Bishop, Edward Laning - with all of whom he had a continuing friendship and whom he influenced even after they became established artists." (Raphael Soyer, Self-Revealment, a Memoir, p.62).

Brook married the artist Peggy Bacon in 1920 and in the same year began writing reviews for the Arts Magazine. In 1924 his association with Juliana Force, founder of the Whitney Studio Club, later the Whitney Museum of American Art, was formed. Brook served as the Assistant Director of the Whitney Studio Club from 1925-1927. Soyer further writes, "Though I began to exhibit comparatively late, I was fortunate. My first pictures, shown in a group exhibition, attracted the attention of the brilliant, young and already established painter, Alexander Brook. He sold one of them for me, and brought me into the Whitney Club." (Soyer, p. 68). Raphael Soyer and Alexander Brook remained close friends throughout their lives and painted each other`s portraits (Alexander Brook`s portrait of Raphael Soyer was exhibited at the Pennsylvania Academy of the Fine Arts in 1933).

Brook and his wife Peggy Bacon were popular artists and prominent socialites in New York in the 1920`s and 30`s (they divorced in 1940). They were also well established with the Woodstock colony of artists. Brook painted numerous celebrities including Katherine Hepburn, Edward G. Robinson and Lincoln Kirstein. His first one-man retrospective was held at the Art Institute of Chicago in 1929 and his first solo gallery show at ACA Galleries in New York in 1930.

Alexander Brook continued to exhibit in major museums and galleries throughout the 1930`s and 40`s and completed his WPA commission for a mural in the Washington, D.C. Post Office in 1939. In 1945, Brook married the abstract artist Gina Knee and the two settled in Sag Harbor three years later. During his later years, he turned increasingly to abstract painting, a style not well suited to his talent. His popularity waned and eventually Brook returned to the representational style for which he is so well appreciated. Alexander Brook was a member of the National Academy of Design and the National Institute of Arts & Letters. In addition, he won numerous prizes, including the Art Institute of Chicago, 1929 (prize), Carnegie Institute, 1930 (prizes); 1939 (prize), Pennsylvania Academy of the Fine Arts, 1931 (Temple Gold Medal), American Section, International Exposition, Paris, 1937 (gold medal), Worcester Museum, 1938 (prize), San Francisco Art Association, 1938 (medal).