[1] He is known for his work on the stress-softening behavior of rubber, a phenomenon now known widely as the Mullins effect.

Leonard was the eldest of 7 children, with his brothers Eric, Kenneth, John, known as Alan, and sisters Sylvia, Muriel and Eugenie.

Mullins graduated from University College London in 1939 BSC (Hons), PhD, DSc He had originally hoped to enter academia, but World War II interrupted his plans and he ended up working in weapons research for the British government.

In 1949, he oversaw the dismantling of the Bayer A.G. rubber labs and pilot plant at Leverkusen, Germany.

In 1950, he joined the physics group of the British Rubber Producers' Research Association.