The commission is currently investigating 88 complaints, according to its executive director, LeeAnn Pelham. Its staff is evaluating another 92 cases to determine if they warrant investigations, Pelham said.
In his letter of resignation to Norman Yee, the president of the Board of Supervisors, Kopp said that the commission's practices were "ineffective and a waste of taxpayer money."
"In short, I have lost confidence in the ability of the Ethics Commission and its staff to achieve the purposes represented to voters and residents two decades ago for its establishment," the letter states.
Kopp said that he would now focus on promoting a local ballot measure, referred to as "Sunlight on Dark Money." The measure would tighten disclosure requirements for independent expenditure committees and place restrictions on fundraising from developers.
The measure was originally pushed by Golden Gate Law School Dean Emeritus Peter Keane, who resigned from the commission last year after the panel failed to place the proposal on the ballot.
Daina Chiu, the chair of the ethics commission, said she was saddened and disappointed to learn of Kopp's resignation.
"I am proud of the great work the Commission and staff have done and are continuing to do to deepen the effectiveness of our reform laws in the city," Chiu said in a statement.
"The commission's mandate is vital and demands a deliberative body that represents a variety of viewpoints. It is unfortunate that San Franciscans will no longer benefit from Mr. Kopp's contributions," she said.
Kopp, 90, has served as a member of the San Francisco Board of Supervisors, a state senator and San Mateo County Superior Court judge. He was responsible for several pieces of legislation aimed at increasing transparency in government and campaign finance.
In 1998 Interstate 380 in San Mateo County was named the Quentin L. Kopp Freeway.
He also has a reputation as a blunt-talking curmudgeon who has levied personal attacks against journalists.
In 2016 he was criticized for harsh language he used in a complaint letter to NPR White House correspondent Tamara Keith (a former staffer at KQED). Kopp focused on the way Keith spoke to another journalist on the radio, describing the interchange as nauseating.
The following year he wrote a similar note to Nuala Sawyer, an editor at SF Weekly.
Last year Kopp sued the U.S. Secret Service for not releasing information on how much taxpayer money was spent on Donald Trump Jr.'s trip to India.
The San Francisco Board of Supervisors will appoint Kopp's replacement. Board president Yee was one of those who endorsed Kopp's appointment to the commission.
"Quentin Kopp has dedicated decades of his life to serving this city and its residents. Anyone who has engaged in public service understands the personal sacrifices that this entails and I thank him personally for his contributions to good government," Yee said in an emailed statement on Wednesday.
KQED's Kate Wolffe contributed reporting to this story.