In his late teens and twenties, he began taking a series of environmental portraits with an Icoflex Zeiss camera, and had the opportunity to photograph many famous Swedes.

He began his professional career in the mid-1940s as a freelance photographer, working frequently for the publisher Åhlen & Åkerlund of Stockholm.

Some of his early photo essays, notably A Midwife in Lapland (1945), Polar Bear Hunting in Spitzbergen (1947), and Fishermen at the Congo River (1948), brought him international attention after publication in Life, Illustrated, Picture Post, and elsewhere.

These advances, combined with very thin endoscopes that became available in the mid-1960s, enabled him to make groundbreaking photographs of living human blood vessels and body cavities.

In an interview published by PBS, Nilsson explained how he obtained photographs of living fetuses during medical procedures including laparoscopy and amniocentesis and discussed how he was able to light the inside of the mother's womb.

"[8] However, Nilsson himself has offered additional explanations for the sources of his photographs in other interviews, stating that he at times used embryos that had been miscarried due to extra-uterine or ectopic pregnancies.

[16] He was awarded the Royal Photographic Society's Progress medal in 1993 'in recognition of any invention, research, publication or other contribution which has resulted in an important advance in the scientific or technological development of photography.