"I've been unfaithful to my wife," he said in a news conference in which the 49-year-old governor ruminated on God's law, moral absolutes and following one's heart. He said he spent the last five days "crying in Argentina."
Sanford, who in recent months had been mentioned as a possible presidential candidate in 2012, said he would resign as head of the Republican Governors Association.
By leaving the country without formally transferring power, critics said he neglected his gubernatorial authority and put the state at risk. It wasn't clear how his staff could reach him in an emergency.
At least one state lawmaker called for his resignation. As a congressman, Sanford voted in favor of three of four articles of impeachment against President Bill Clinton, citing the need for "moral legitimacy."
Sanford described the woman who lives in Argentina as a "dear, dear friend" whom he has known for about eight years and been romantically involved with for about a year. He said he has seen her three times since the affair began, and his wife found out about it five months ago.
A newspaper published steamy e-mails between Sanford and the woman. He did not identify her, nor did he definitely say.
"What I did was wrong. Period," he said. His family did not attend the news conference, and his wife Jenny Sanford said she asked the governor to leave and stop speaking to her two weeks ago. The governor said he wants to reconcile, and his wife's statement said her husband has earned a chance to resurrect their marriage.
"This trial separation was agreed to with the goal of ultimately strengthening our marriage," she said.
Sanford did not answer directly whether the relationship with the Argentinian woman was over.
He did say, "I had, to the people of South Carolina, based on my boys, based on my wife, based on where I was in life, based on where she was in life, a place I couldn't go and she couldn't go."
Sanford denied instructing his staff to cover up his affair, but acknowledged that he told them he thought he would be hiking on the Appalachian Trail and never corrected that impression after leaving for South America.
"I let them down by creating a fiction with regard to where I was going," Sanford said. "I said that was the original possibility. Again, this is my fault in ... shrouding this larger trip."
Questions about Sanford's whereabouts arose early this week. For two days after reporters started asking questions, his office had said he had gone hiking on the trail.
Cornered at the Atlanta airport by a reporter from The State newspaper, Sanford revealed Wednesday morning that he had gone to Argentina for a seven-day trip.
When news first broke about his mysterious disappearance, Jenny Sanford told The Associated Press she did not know where her husband and father of their four sons had gone for the Father's Day weekend. She said he needed time away to write.
Sanford emerged Wednesday afternoon at a news conference, where he mused openly of his love of hiking and how he used to guide trips along the Appalachian Trail, and eventually tearfully apologized to his wife, his staff and his friends - but without yet saying what he was apologizing for.
"I hurt a lot of different folks," he said, occasionally choking up throughout the news conference that lasted about 20 minutes.
With those watching still wondering what he was admitting, Sanford said: "The odyssey that we're all on in life is with regard to heart."
Excerpts of e-mail exchanges between the governor and his mistress were published online Wednesday by The State. The governor's office wouldn't discuss the e-mails with The Associated Press, but told The State it wouldn't dispute the authenticity of the messages.
One from the governor read: "I could digress and say that you have the ability to give magnificent gentle kisses, or that I love your tan lines or that I love the curve of your hips, the erotic beauty of you holding yourself (or two magnificent parts of yourself) in the faded glow of the night's light - but hey, that would be going into sexual details."
In another: "In the meantime please sleep soundly knowing that despite the best efforts of my head my heart cries out for you, your voice, your body, the touch of your lips, the touch of your finger tips and an even deeper connection to your soul."
Several residents said they were disappointed in Sanford.
"He shouldn't have lied to us. He should have been up straight," said college student Gerald Walker, 19, in downtown Columbia. "It's very embarrassing for someone in a leadership role that we are supposed to respect, especially me being a young guy."
Glenn Mitchell, of Columbia, said he felt Sanford's absence showed a lack of concern for the state.
"He left the state unattended," said Mitchell, 54, out of work recuperating from surgery. "He just hasn't been there for us."
But Warren "Cubby" Culbertson, a longtime friend who said he has been counseling Sanford, said the governor was accepting responsibility for his actions.
"Any man can fall. But it takes a real man to get up and honestly, from his heart, confess that he was wrong," Culbertson said. "And he's going to try to change."
Others were less forgiving. State Rep. Todd Rutherford, D-Columbia, called for Sanford's resignation.
"There is nothing left to save," Rutherford said. "There is no reason for him to remain as governor."
Sanford, a former three-term congressman, was elected governor in 2002. He has more than a year remaining in his second term and is barred by state law from running again.
Sanford was elected chairman of the Republican Governors Association this year after he helped raise a record $10.6 million at the group's 2008 annual dinner to help elect GOP governors. The association said Mississippi Gov. Haley Barbour would assume the duties as chairman.
The libertarian-leaning Republican was seldom a firebrand. But he was known for salting tales of family life into policy discussions.
He criticized the $787 billion federal stimulus law and efforts by legislators to claim a share of it by saying in tough times a family would sit around the table and find ways to cut spending.
His vocal battle against the Obama administration over the stimulus money won praise from conservative pundits, but ultimately, a state court order required him to take the money.
Jenny Sanford, a millionaire whose family fortune comes from the Skil Corp. power tool company, has been central to Sanford's political career. She ran his congressional campaigns and his first race for governor. She was an almost daily fixture at senior staff meetings, and often could be seen driving a minivan away from the Statehouse in the mornings.
The two met when Sanford, who has an MBA, was trying his hand on Wall Street. She was working at a brokerage house when he entered a training program.
As governor, Sanford has had seemingly endless run-ins with the GOP-dominated Legislature, once bringing pigs to the House chamber to protest pork barrel spending. He also put a "spending clock" outside his office to show how quickly a proposed budget would spend state money.
Sanford's announcement came a day after another prominent Republican, Sen. John Ensign of Nevada, apologized to his GOP Senate colleagues after revealing last week that he had an affair with a campaign staffer and was resigning from the GOP leadership.
AFFAIR APOLOGY: South Carolina Gov. Mark Sanford apologized to his wife and four children at a press conference where he admitted taking a secret trip to Argentina to visit a woman with whom he'd been having an affair. He said the woman was a good friend for eight years but the romantic relationship started about a year ago.
MYSTERY TRIP: Sanford's trip to Argentina came to light after reporters started asking questions about his whereabouts earlier this week. His staff had said he was hiking on the Appalachian Trail. He said Wednesday that's what he told them.
TROUBLE FOR THE GOP?: Sanford said he'll resign as head of the Republican Governors Association, and his admission came a day after prominent Republican Sen. John Ensign of Nevada apologized for an affair with a campaign staffer. GOP governors were once thought to be President Obama's most credible adversaries, but that's not turning out to be the case.