When President Barack Obama announced in 2013 his decision to tap Mary Jo White to run the SEC, he set high expectations that the former high-profile prosecutor could repair the image of an agency tarnished by the financial crisis and a series of shocking investment scandals. “You don’t want to mess with Mary Jo,” Mr. Obama said at the time.

But during her tenure, Ms. White has struggled with political conflict from both Democrats and Republicans that slowed her agenda, and she ended up the latest target for critics who argue that Wall Street still gets off too easily when it violates the law.

• The SEC under Ms. White pursued record numbers of enforcement cases, including dozens of insider-trading claims, yet had no marquee case that made critics forget its failure to root out fraud before the crisis.
• The SEC wrote more than 40 regulations but didn’t complete all of the rules assigned it by the 2010 Dodd-Frank financial overhaul.
• The SEC feuded with other regulators who saw gaping holes in its supervision of the $15.7 trillion mutual-fund industry.

“She had political fights to engage in that obviously impacted her initiatives and her success in office,” said former SEC Chairman Arthur Levitt.

For all the Democratic complaints that Ms. White didn’t do enough on regulation and enforcement, her tenure is likely to be a high-water mark for both, at least for the foreseeable future. Donald Trump’s victory opens the door to a Republican successor who could veer toward a deregulatory approach after six years of passing stricter rules required by Dodd-Frank. Ms. White’s successor also could pull back on the scrutiny that the agency has applied to mutual funds, hedge funds and private-equity firms in recent years.

Unlike some of her predecessors, Ms. White leaves the agency without a major scandal or controversy dogging her tenure. Her predecessor, Mary Schapiro, left in 2013 after the commission rejected her plan to restructure how the riskiest money-market mutual funds work. Former Chairman Christopher Cox left in 2009 as the SEC was taking blame for failing to foresee the credit crisis.