Son of George Eckfeldt, nephew of Jacob Eckfeldt, grandson of Adam Eckfeldt. Married. He had been fired from the coining department for stealing. He was employed as a night watchman for the mint.

It is believed he was responsible for a weird series of clashed dies in 1857. These include the 1857 flying eagle cent clashed with a seated liberty quarter reverse, 1857 flying eagle obverse clashed with seated liberty half obverse and 1857 flying eagle obverse clashed with $20 liberty gold obverse.

In 1858 he conspired to strike at least 5 of the class II 1804 silver dollars with plain edges. He sold them around Philadelphia for $75 each. Mint authorities recovered them and concealed all but one that remained in the mint collection. It was overstruck on an 1857 Swiss shooting thaler. He produced restrikes of the 1827/3 quarters.

Theodore Eckfeldt has become known as the "Midnight Minter.""