After losing the presidency to Socialist Francois Hollande in 2012, Sarkozy made a storming comeback in September, although recent polls showed his popularity among members of the crisis-torn UMP wavering.
Sarkozy won 64.5 percent of the vote. Analysts had said before the vote that he needed at least 70 percent to see off rivals and become the party's undisputed champion for the presidential battle in 2017.
He scored 85 percent when he last won the leadership, in 2004, AFP reported.
Alain Juppe, a UMP heavyweight who has said he wants to be its presidential candidate in 2017, was not backing down.
"I am ready to help him of course if we take the course I have set out, rallying the right and the centre," he told reporters after the result.
"We will see for the (presidential) primaries. That's not the subject. The opposite of vigilance is what? It's going to sleep? I am not going to go to sleep."
Francois Fillon, Sarkozy's other main potential rival for the UMP presidential ticket, took a similar line.
"Unity does not mean submission," he said in a statement. "For my part I will defend my convictions."
Sarkozy said the campaign had been "dignified" and took comfort in what he said was a record turnout, calling it in comments on his Facebook page "the best response to two years of internal quarrels and divisions".
The 24-hour online vote was slowed on Friday evening by a cyber attack on the election website, but in the end more than 58 percent of its members had cast a vote, a bigger turnout than in past elections, and party officials declared it fully valid.
With Hollande's ratings sent to record lows by tax rises and a failure to tackle unemployment, the opposition conservatives should be flying high. Economic data last week showed the jobless rate at the latest record high and consumer spending in the doldrums.
But the UMP has long been riven by leadership squabbles, and both it and its new leader are mired in a legal inquiry into alleged funding irregularities.