Only three friends were present for the ceremony Thursday at the registry office in the British colony of Gibraltar, The Sunday Telegraph reported. Sir Mark Thatcher married Sarah Russell, who was divorced last year from Lord Francis Russell, son of the Duke of Bedford.
Sir Mark Thatcher said he told his mother, now Baroness Thatcher, about the impending wedding and described her as being pleased with the news.
"We were determined to keep the day for ourselves and only had three close friends at the ceremony and they kept it a secret for us," he told the Telegraph. "The last thing we had wanted to do was to have it turned into any kind of media event."
The government of Equatorial Guinea this week issued a warrant for Sir Mark Thatcher's arrest. He said in an earlier interview that he felt "pretty relaxed" about it.
The groom has two teenage children by his first marriage. He has been living in Gibraltar recently, although reports have said he might return to Britain to be closer to his mother.