His vice presidential announcement was initially slated to take place this morning at a news conference in New York City, but that was postponed out of respect for Thursday's attack in Nice, France.
Pence followed up by showing he was on board in two ways: first by formally filing the paperwork showing that he does not intend to run for his gubernatorial re-election, and secondly by tweeting about it.
Trump and his campaign team have made repeated announcements this week that he would be formally announcing the name of his vice presidential pick on Friday, at the end of a week of detailed speculation over who he would choose.
Earlier Thursday, Trump asked Mike Pence to be his running mate, and the Indiana governor accepted, a source with direct knowledge told ABC News. Hours after that reported offer, however, Trump told Fox News that he had not made a "final, final decision."
In an interview this morning on Fox News, Trump campaign chairman Paul Manafort said Trump had reached a decision on his running mate, but that it would not be official until announced.
"I don’t know that he’s having second thoughts," Manafort said when asked. "But he hasn’t announced it yet."
Pence officially threw his support behind Trump in June.
"Now that the primaries are over, it's time to come together," Pence said at the Indiana GOP convention. "It's time to come together around the people who were the people’s choice."
In his state's primary, Pence chose to back Texas Sen. Ted Cruz over Trump. His endorsement, however, was lukewarm; Pence still praised Trump for taking a "strong stand for Hoosier jobs" and said he was grateful for Trump's voice in the national debate. Trump in turn called Pence "terrific."
Pence was once seen as a potential presidential hopeful himself, but his prospects dimmed in the spring of 2015 amid a nationwide controversy over his state’s “religious-freedom” law, seen by many as targeting gay and lesbian residents.
The law, signed by Pence, was worded as a protection of “religious freedom” for people and corporations, but both social conservatives who supported it and gay-rights advocates who opposed it maintained it would allow businesses to deny services to gay people if employees felt that would infringe on their religious beliefs.
Pence defended the law in an interview with ABC News' George Stephanopoulos, saying it was “not about discrimination,” while repeatedly declining to say whether the law would mean florists, for instance, could refuse to provide their services for gay weddings. Indiana does not have other laws on the books protecting gays and lesbians from discrimination. The governor ultimately signed a revision to the law, as corporations threatened to divert business from Indiana, but not before a national backlash.
In choosing Pence as his No. 2, Trump has the potential to broaden his voter base. He will also be adding a social conservative who can appeal to white evangelical Christians, among whom Trump already polls well -- with 68 percent support in ABC News' latest survey.
Pence’s relationship with Washington insiders stands to help Trump bring in more donors, growing a fundraising operation that only got off the ground last month, after the billionaire financed his primary campaign largely with loans from his private wealth.
ABC News' John Santucci contributed to this report.