A positive outcome "was anything but assured," Geithner said in an op-ed published Wednesday in The Washington Post.
"President Obama took office faced with an industry that was burning and had to determine whether additional government support made sense," he wrote, referring to $17 billion given to automakers during the waning months of the George W. Bush administration.
But Geithner said jobs were at stake -- up to 1 million of them counting supply companies, dealerships and a host of other businesses that rely on the automobile.
Two years after billions of dollars were spent to prop up the industry, Chrysler has posted five consecutive quarters of profit, Geithner said.
Combined, "GM and Chrysler have returned more than 50 percent of the government's investment. The industry is mounting one of the most improbable turnarounds in recent history," he said.