He wrote holding public office has put strains on his family. But he has also come under fire about campaign finances.
"The people of District 41, and all citizens of North Carolina, deserve representatives who can make clearly-focused decisions on their behalf," Harrell wrote in the letter. "With the recent turbulence in my personal life and continued speculation about my campaign expenditures, I do not feel that I can provide the high standard of representation that my constituents expect and deserve."
The news follows court documents filed two months ago by Harrell's estranged wife, Melanie Dupon, that stated he was "engaging in an adulterous affair with his mistress with whom he is still involved."
His wife of 18 years said she wanted custody of their sons and financial support. She said Harrell was reckless with money and behavior and that he "would stay out until the early morning hours, spending time with other women and engaging in excessive use of alcohol."
In July, Harrell told Eyewitness News the allegations of affairs and heavy drinking were new to him and his political future was the least of his concerns and he was focusing on his family.
Sunday evening, Speaker Hackney released a statement in response to Harrell's resignation.
"Ty is to be commended for putting his children first as he works through the problems at hand," Hackney said. "Stepping down now shows great respect for our House of Representatives and the people of his district. I wish him well."
Harrell said in the letter that once he has a better balance between his personal life and professional responsibilities, he might find another way to serve the public.