Although he was defeated in the overall election, the YouGov/ElectionData poll found that Smith also won among 18-24 year-olds and Scottish party members.
The survey found that 63% of pre-2015 party members had backed the Pontypridd MP, to just 37% for Corbyn.
But the current leader had a huge lead among the tens of thousands who have joined the party since Ed Miliband quit, with 74% of them backing him to just 24% for Smith.
Among those who had joined since Corbyn became leader, a massive 83% said they had voted for him in the 2016 leader election, and 15% voted Smith.
The exit poll was among members only and did not include ‘registered supporters’ or trade unionists and other affiliates.
ElectionData pollster Ian Warren, who commissioned the poll, said the exit survey result was “spot on” when compared to the final result.
The findings underline claims from Smith supporters and several Labour MPs that the party membership has radically changed since the general election.
The YouGov data found that Labour is a very different party from even last September’s leadership contest, with 20% of current members admitting they didn’t vote for the party in the 2015 general election.
Of the non-Labour 2015 voters, 78% voted for Corbyn.
Smith had a 55-45 lead over Corbyn among younger voters , a stark contrast with the perception that the Labour leader has won round the under-24s during his popular campaign of rallies across the country.
Warren, whose Election Data consultancy commissioned the exit poll, is a former polling adviser to Ed Miliband.
The exit poll found that 59% of Labour members said they had voted for Corbyn and 41% for Smith, but once weighted it forecast a 65% victory for the leader and 35% for his challenger.
According to the exit poll, Scotland was the only area where Smith came out on top, with 58% to his rival’s 40%.
Corbyn won narrowly in London - where Mayor Sadiq Khan has backed his rival - with a 53% to 47% victory
Corbyn had a large lead among those Labour members who voted Leave in the EU referendum, by 75% to Smith’s 23%.
The Labour leader also won a huge mandate among women Labour members, with 64% backing him, compared to just 35% preferring Smith. In contrast, Smith did better among men (46%) but still lost to Corbyn (53%).
Corbyn’s campaign have long claimed he is very popular with women in the party, and Smith faced a series of sexism rows during the campaign, including his remark that he wanted to “smash” Theresa May back on her heels.
YouGov, which came under fire along with other pollsters for its failures in the 2015 general election and the EU referendum, was praised for its accurate call on the Labour leadership
The pollsters had in a survey in August putting Corbyn on 62% to Smith’s 38%, a finding which some MPs felt changed the narrative of the race.
The defeated challenger today said: “I am humbled by the more than 193,000 members, supporters and trade union members who have put their faith in me.”
Smith pleaded with pre-2015 party members to stick with Labour despite his defeat.
“Above all, despite present divisions, we have to stick together in for the long term. I call on those party members disappointed by the result and tempted to look elsewhere to stay with Labour and to stay involved. Let’s work together to renew this movement and take the fight to the Tories,” he said.
Smith said he would stay in Liverpool until Sunday but would then be “returning home to Wales to spend some precious time” with his family.
He will not be at the conference to hear John McDonnell’s speech on the economy on Monday or Corbyn’s main keynote address on Wednesday.
Another YouGov poll yesterday found that 71% of former Labour voters blamed Corbyn’s lack of Prime Ministerial qualities.
Meanwhile, Corbyn’s critics within the party also urged members not to desert it, deploying a #StayInLabour hashtag on Twitter.