Harman, who held the 36th District seat for six years before giving it up to run for governor in 1998, received the congratulatory phone call from the one-term congressman on Thursday, more than a week after Election Day.
"We've counted enough absentee ballots that it seems appropriate at this time to concede," Kuykendall later said.
The race was among the most heated and expensive in California, with spending expected to top $4 million. During the campaign, Kuykendall called Harman a carpetbagger with few ties to the district. Harman criticized the incumbent for voting against a plan to add prescription drug benefits to Medicare.
After conceding, Kuykendall said the contest was particularly difficult because the two had been friends for years. He promised to work with Harman for a smooth transition.
"This was a very tough campaign for both of us because of that friendship," he said.
"I hope we can resume our relationship of the past," Harman said minutes after receiving Kuykendall's phone call. She was in Washington attending a congressional orientation session.
Though some ballots have yet to be counted, it would be nearly impossible for Kuykendall to close the gap of nearly 4,000 votes. As of Friday, Harman had 48.4 percent of the vote to Kuykendall's 46.8 percent.