[citation needed] In 1955 his work, along with that of his wife Caroline Hebbe, was selected by Edward Steichen[1] for the world-touring Museum of Modern Art exhibition and book The Family of Man,[2] seen by 9 million visitors.
[3] Hammarskiöld was the last surviving member of the group Tio fotografer (Ten Photographers), which formed in 1958 and was influential in Swedish photography for decades as the illustrations agency Tiofoto.
The group included Sten Didrik Bellander (1921–2001), Harry Dittmar, Sven Gillsäter (1921–2001), Rune Hassner (1928–2003), Hans Malmberg (1927-1977), Pål-Nils Nilsson (1929–2002), Georg Oddner (1923-2007), and Lennart Olson (1925–2010).
Examples of Hammarskiöld's portraiture are his humorous photographs of pop artist Claes Oldenburg in 1966 carrying a huge tube of toothpaste, Pontus Hultén shouting and dressed in an over-decorated coat in 1984, and his profile portrait of Professor Ragnar Josephson in 1965.
In 2009, a selection of seventy of Hammarskiöld's portraits was on display at the National Museum in Stockholm.