Slim was raised in New York and New Hampshire and began his photography career while in the Army and before long was appointed the official photographer of the United States Military Academy at West Point. When war broke out, he was sent as a combat photographer for Yank magazine and saw action from North Africa to the Middle East to Europe, witnessing the fall of Tobruk and the liberation of Rome, and he received a Purple Heart.
After leaving the army, Slim became a freelance photojournalist and through his Yank connections headed for his first civilian tour of duty in Hollywood. Slim photographed the Hollywood elite – Humphrey Bogart, Lauren Bacall, Tony Curtis and Janet Leigh to name but a few. When Life magazine opened a bureau in Rome, Slim relocated and it was in the Eternal City, destination of an international array of celebrities and aristocrats, that Slim began in earnest his lifelong project documenting the Beautiful People.