Lewandowski and Barry Bennett, who formerly managed Ben Carson's unsuccessful bid for president, are teaming up on the new venture, called Avenue Strategies.
The pair announced the creation of the firm on Wednesday, as Trump was expected to begin announcing more of his White House staff. Whether Lewandowski would be included in the West Wing had been an open question during the transition.
It's unclear if Lewandowski had been offered anything in the Trump administration. He had met with Trump's influential son-in-law, Jared Kushner, on Monday, a source familiar confirmed to CNN. The transition did not respond to a request for comment or answer whether Lewandowski had been considered for a position in the White House.
But Lewandowski said he would still support Trump through his new venture.
"I will always be Presidnt-elect Trump's biggest supporter," Lewandowski said in a statement. "After considering multiple opportunities within the administration, I informed him and his team I think I can best help him outside the formal structure of the government. I very much look forward to doing that every day."
Bennett told CNN that Avenue Strategies will be "a full service government affairs and political consulting shop to, from the outside, help pursue the Trump agenda." But Bennett was clear the organization would not be an "extension of the campaign" or political operation the way Obama's campaign arm became the group Organizing for America.
It is not unusual for former campaign officials to make money in the private sector after a campaign, rather than join the administration. A wide range of potential clients will pay a premium to former high-level advisers who can help navigate the new White House.
Lewandowski managed Trump's campaign through the primary, but was fired in June after controversy and intervention from Trump's advisers and family members, who urged the real estate mogul to overhaul his campaign for the general election.
After Carson's earlier withdrawal from the primary, Bennett had also been a senior adviser to the Trump campaign, but was pushed out after Lewandowski's replacement, Paul Manafort, took over. Manafort was replaced later in the summer by Kellyanne Conway, who was the manager that led Trump's campaign through its victory.
Lewandowski worked as a political commentator supporting Trump for CNN through the election, after which he resigned. He had been engaged with the transition effort post-election, but had no formal role unlike the rest of Trump's inner circle.
The firm will take clients starting January 1, Bennett said, though they had none yet to announce.
Before announcing their venture, Bennett said Lewandowski first spoke with the President-elect, incoming White House chief of staff Reince Priebus and senior adivser Steve Bannon to inform them of his decision to not take a job in the administration and instead open this consulting firm.
A source familiar said Lewandowski would only make such a move with Trump's "blessing," and said Lewandowski was getting tired of infighting. He had few allies inside the transition team.
"He's always been the one that was going to get screwed from the infighting," the source said.
The new venture could be highly lucrative for Lewandowski and Bennett, who will have rich contacts and influence within the Trump administration.
The office listed in the firm's press release is on a block adjacent to the White House grounds.