Once he returned, the Cavaliers were headed to the Eastern Conference finals.
James, who had a long stint on the bench with foul trouble, scored 23 points, and the Cavaliers advanced to the conference finals for the first time in 15 years by beating the New Jersey Nets 88-72 on Friday night.
Cleveland won the series 4-2 and heads to Detroit on Monday night for its third appearance in a conference finals and the first since 1992. James, who had eight rebounds and eight assists, needed only four years to get the Cavs into the NBA's final four.
"It's a great feeling," James said. "This is one of the best feelings I've ever had as a basketball player."
He got plenty of help in the clincher from reserves Donyell Marshall and Daniel Gibson, and the Cavs needed it because their superstar spent most of the third quarter on the bench in foul trouble while Cleveland managed just eight points. New Jersey cut what had been a 22-point deficit to one, but with James back for the fourth quarter, the Nets never could take the lead.
"I think it was tough on every Cavs fan to watch that third quarter," James said. "It's one of those quarters that we didn't want to have."
They survived it because Marshall scored 18 points and Gibson added eight. They had combined for 26 points total in the first five games of the series.
Jason Kidd had 19 points, 12 rebounds and eight assists for the Nets, who stayed alive with a victory at Cleveland in Game 5 and were hoping to become the ninth NBA team to come back from 3-1 deficit to win a series.
Instead, they stalled in the conference semifinals for the second straight year.
Richard Jefferson scored 16 points and Mikki Moore added 14, but leading scorer Vince Carter had only 11 and heard boos in what could have been his last game for the Nets. He can become a free agent this summer.
"We had a great team and we're definitely built to go far," Carter said. "And when you fall short it's always disappointing. It's tough, it's always tough."
Cleveland led by 15 points early in the third quarter, but the Nets stormed back when James went to the bench with his fourth foul and the Cavs leading 59-46 a little more than four minutes into the period.
"Oh golly. Shucks, what are we going to do next?" Cavs coach Mike Brown said of his thoughts when James picked up No. 4. "I think that's how it went.
"I felt that if we could keep it close going to the fourth, we'd find a way to win, especially with him on the floor."
Kidd had two baskets to trigger a 14-2 run to end the period. The Nets outscored the Cavs 22-8 in the quarter - Cleveland just missed the worst third quarter in NBA playoff history - and closed to 61-60 heading to the fourth.
The Nets were down only one after Moore's three-point play with 9:38 to play, but Gibson and Marshall made consecutive 3-pointers to push the lead to 70-63 with 7:45 left. James made a jumper less than a minute later for his first points since the first quarter, and Marshall - 0-for-7 in Game 5 - buried another 3 to make it 75-66 with 5:47 remaining.
"We got down to one and I was spent in the sense of I shot an airball, that's when I knew I was a little tired," Kidd said. "But we put ourselves in that hole and at that point I was just hoping somebody would pick it up from where I left."
Marshall was 6-for-10 from 3-point range on his 34th birthday.
"Actually after the last game I went home and laughed," Marshall said. "After that game, with today being my birthday, LeBron told me they were going to get it done for me today."
With that, Cleveland prevented another second-round heartbreak. The Cavs led Detroit 3-2 last year before dropping the final two games.
The Cavs missed their first chance to clinch with an 83-72 loss Wednesday night in Cleveland. James bruised and cut his knee chasing a loose ball in the final minute of that game, but was quickly cleared to play and said he was fine before Friday's game.
James scored 14 points in the first quarter, but stayed stuck on that total while the Nets nearly made the comeback.
"He made all the right plays there in the fourth," Kidd said. "He made all the right plays and guys made shots coming down the stretch."
NOTES: James said before the game that his pregnant girlfriend was fine, two days after she was taken to the hospital during Game 5 in Cleveland. Asked if the couple had a name for the baby, due next month, James said: "She does, but I don't." ... The fewest points in a third quarter is six by Atlanta at Boston on May 6, 1986. ... Cleveland guard Eric Snow needed three stitches for a cut on his left elbow.