The development was first reported by MSNBC and BuzzFeed. McMullin could not be immediately reached for comment.
McMullin will be pitched as an option for conservatives who have never warmed to Trump and who remain adamantly opposed to Democrat Hillary Clinton, although his chances of success in the Nov. 8 election are near zero.
"He is running, first and foremost, out of a deep love for this country, and because he understands the true brand of American leadership that is required to be Commander-in-Chief," an email sent by his campaign to supporters said.
Trump was formally anointed his party's nominee last month after beating 16 rivals in the primary contests. But many Republicans have been concerned about Trump policies such as his proposal to put a temporary ban on Muslims entering the country and about his free-wheeling, often insulting rhetoric.
McMullin has never held public office and is unknown to American voters. In addition to also lacking a quick source of campaign cash, he will face immediate hurdles to try to get his name on enough ballot papers to make himself a serious candidate.
Texas, for example, requires third-party candidates to get more than 79,000 signatures from residents who did not vote in either the Republican or Democrat primary. And the deadline for that was in early May.
Deadlines to get on the ballots have also lapsed for the large states of North Carolina, Illinois and Florida.
The best McMullin could likely hope for would be to simply play spoiler to Trump in a handful of states, eating away at the New York real estate developer's ability to win states that are generally reliably Republican.
McMullin would join two other third-party hopefuls - Gary Johnson, who was nominated by the Libertarian Party, and Jill Stein, who will represent the Green Party. In recent polls that included Trump, Clinton, Stein and Trump, the third-party candidates have both struggled to get above 10 percentage points each in the polls.
McMullin has been a frequent critic of Trump on social media, calling him an authoritarian and criticizing his stance on civil rights as well as his refusal to release his tax returns.
McMullin most recently served as the House Republican Conference's chief policy director and has worked in Congress since 2013, according to a profile on LinkedIn. He previously spent 11 years as an operations officer for the Central Intelligence Agency.
The House Republican Conference, in a statement, said McMullin no longer worked there, effective Monday morning.
Trump, meanwhile, was seeking to reset his campaign with a big economy policy speech on Monday after a rough patch last week. He was widely criticized, including by some senior Republicans, for engaging in a public dispute with the parents of a Muslim American soldier killed in Iraq.