Immigration reform will happen, and the public has grown far too pessimistic on its prospects, Treasury Secretary Jack Lew told CNBC on Wednesday.

Lew said he believes lawmakers will come to a resolution on reforms languishing in Congress for the past year. After months of false starts, Lew said, there’s now enough consensus among lawmakers to act.

“I personally believe there will be a resolution on immigration reform,” Lew said on CNBC’s “Squawk Box.” “And we’re all going to have to work hard to make that happen.”

Lew declined to attach his own economic value on reforms that would create a new path to citizenship for millions of undocumented immigrants.

He deferred to the nonpartisan Congressional Budget Office, which last year found that immigration reform would increase real GDP by 3.3 percent in 10 years and help drive down the federal debt.

Read More Jack Lew: ‘Main Street’ hasn’t been made whole yet

Putting more workers “on the books” would increase the tax base and help fund cash-strapped programs such as Social Security and Medicare, Lew said.

“It’s the right thing to do,” Lew said. “It’s the history of our country, how we’ve grown as a country. It’s how we’ve prospered.”

—By CNBC’s Jeff Morganteen.