Leonard was born on the 24th of November, 1878 in Middleton, Delaware, at at the age of 16, he enlisted in the US Navy in Philadelphia as an Apprentice, First Class. Prior to the Spanish-American War, Leonard was on board the USS Richmond, Vermont, Constellation, Alliance, Puritan, Brooklyn, and Texas, but it was his actions on board the USS Marblehead that would earn him the Medal of Honor. The citation reads:

On board the USS Marblehead during the operation of cutting the cable leading from Cienuegos, Cuba, May 11, 1898. Facing the heavy fire of the enemy, Chadwick set an example of extraordinary bravery and coolness throughout this period.

After the war, Leonard took part in the Second Boer War, which was a war between the British Empire and the South African Republic with the Orange Free State. It was here that he helped save wounded men and weaponry while under fire, for which he received the Queen's Scarf. This was a personally crocheted scarf from Queen Victoria and was only awarded to eight people.

Leonard moved to Boston after the wars and sold insurance. He later worked in the leather industry but in 1923, an accident would leave him totally disabled. Leonard B Chadwick died on the 18th of May, 1940 at the age of 61 and is buried in the Mount Hope Cemetery in Mattapan, Massachusetts.