Bush's endorsement comes as Newt Gingrich says he will scale back his campaign.
Bush, president from 1989-93, endorsed the front-running Republican presidential hopeful in Houston after the two met privately. Bush had decided in December to back Romney, NBC News reported.
Former first lady Barbara Bush already backed Romney and recorded automated telephone calls in his behalf that were used ahead of the Super Tuesday primary elections in Ohio and Vermont March 6. She had said March 5, "I think it's been the worst campaign I've ever seen in my life."
The former president made his announcement during a joint appearance with Romney in Houston.
"Barbara and I are very proud to fully and enthusiastically endorse and support our old friend, Mitt Romney," Bush told reporters. "He's a good man. He'll make a great president and we just wish him well."
The Bushes' son, former Florida Gov. Jeb Bush, endorsed Romney, a former Massachusetts governor, after he won the Illinois primary last week.
Another son, former President George W. Bush, has not expressed a preference in the presidential primary.
The elder Bush, who turns 88 in June, signaled his approval of Romney in a December interview with the Chronicle, calling Romney "the best choice for us" after earlier telling CNN Romney was a "reasonable guy, a conservative fellow" who would make a "very good president."
Becker told the Chronicle Wednesday Bush was a Romney supporter from the start, but waited to make an endorsement out of respect for Texas Gov. Rick Perry, who also sought the Republican presidential nomination.
Perry ended his campaign Jan. 19.
The Texas Republican primary is to be held May 29.
Santorum -- a former U.S. senator from Pennsylvania whose lead among likely GOP voters ahead of the April 24 Pennsylvania primary has shrunk to a statistical dead heat with Romney -- said he would consider being Romney's vice president.
"I'll do whatever is necessary to help our country," he told the Christian Broadcasting Network Monday, adding he considered this year's presidential campaign "the most important race in our country's history."
Romney later told NBC's "The Tonight Show with Jay Leno" he was "happy with Santorum] saying he'd like to be part of an administration with me, nothing wrong with that -- if he's the VP that's better."
Gingrich insisted Wednesday that laying off a third of his staff, including top aide Michael Krull, was exactly what his campaign needed to ensure he could continue running.
"We're staying in," the former U.S. House speaker from Georgia told WTOP-AM, Washington. "That's exactly why we're downsizing and doing what we need to to be able to stay in."
Casino billionaire Sheldon Adelson, who gave about $16.5 million to the Gingrich "super PAC" Winning Our Future, said he believed Gingrich was "at the end of his line."
The Las Vegas Sands Corp. chairman and chief executive officer told a group outside his Venetian Resort Hotel Casino in Las Vegas Gingrich "can't get anywhere near" the 1,144 delegates he needs to capture the Republican presidential nomination -- he has an estimated 135 -- and it's "unlikely there'll be a brokered convention," The Jewish Journal of Greater Los Angeles reported.
A brokered convention happens when no presidential hopeful wins enough delegates in primaries and caucuses, and the nomination is decided through a process of alternating political "horse trading" and additional re-votes.
Gingrich told WTOP Romney doesn't have the 1,144 delegates either, and until he does, "I owe it to the people that have helped me over the past year to represent their views and their values."
"Romney has to earn this," he said. "It's not going to be given to him. And we have every right to run."
U.S. Rep. Ron Paul of Texas vowed Wednesday night to stay in the race too.
"I'm staying in to find out who is going to be winning," he told WBAL-AM, Baltimore, before addressing about 2,000 students at the University of Maryland in College Park.
"There is no way to cancel all of the delegates we have accumulated, and we are going to accumulate a lot more, and they're going to go to the convention," he said.
Maryland, the District of Columbia and Wisconsin hold their primaries Tuesday.