Mary Todd met William "Billy" Herndon, a young Springfield lawyer, shortly after she arrived from Lexington KY.  Herndon, much like most of Springfield's young men were mesmerized by Mary Todd's beauty, culture, and intellect.

Unfortunately, Herndon got on the wrong side of Mary Todd,  However, Herndon did not purposefully intend for this to happen.

At one of the dances, Herndon asked Mary Todd to dance.  During the dance Herndon sought to compliment Mary Todd by saying " you dance like a serpent", meaning of course she is a great dance partner.

Herndon thought of a serpent as a snake who slithered gracefully in the grass. Nevertheless, Mary Todd was aghast.  How dare he compare me to a snake she would say later.

Mary Todd's animosity toward Herndon never went away.

She couldn't understand why Abraham would ask Herndon to be his law partner.  Herndon was younger and almost the completed opposite of Lincoln.  Furthermore, Mary Todd felt that Lincoln felt sorry for Herndon, a man of no money although he had a better law education than Lincoln.

Herndon also loathed the Lincoln ‘brats,’ who romped into the office, scattered papers helter-skelter, upset inkwells and stacks of books, and left the place a shambles. He would have prescribed and taken pleasure in administering a lusty larruping; but the indulgent father seemingly thought nothing of it.”1 

Herndon and Mrs. Lincoln formed a mutual non-admiration society.

He was not much more complimentary towards the Lincoln children: 

“I have felt many and many a time that I wanted to wring their little necks, and yet out of respect for Lincoln I kept my mouth shut. 

Lincoln did not note what his children were doing or had done.”  2

After Lincoln's assassination, Mary Todd Lincoln had to deal with Herndon as did Lincoln's oldest son, Robert.

In 1873, Mary Todd Lincoln and Herndon got into a public quarrel when Herndon alleged Abraham Lincoln was an atheist and even worse that Lincoln never really loved Mary Todd but he was still in love with the deceased Anne Rutledge, his first real love.  .

Both Mary Todd and Robert undertook guarding Lincoln's reputation.  

Herndon certainly wasn't consistent in his writing about his former law partner.  He eventually wrote a book with Jesse W. Welk titled "Herndon’s Lincoln: The True Story of a Great Life, 1889".

One one hand Herndon wrote that he knew of no man with greater charity than Lincoln.  On the other hand, Herndon stated numerous negative thoughts which were hurtful to the Lincoln family.

Credits:

1 Benjamin P. Thomas, Portrait for Posterity: Lincoln and His Biographers, p. 11.

2 Emanuel Hertz, The Hidden Lincoln, p. 105 (Letter from William H. Herndon 

      to Jesse W. Weik, November 19, 1885).

 Lewis E. Lehrman, Mr. Lincoln and Friends