Jeremy Corbyn has extended his influence over Labour's governing body following elections to the National Executive Committee.
Jon Lansman, the founder of the Corbyn-supporting Momentum group which wants more grassroots say over policy, was among three new members elected.
He hailed the result as a victory for "21st Century socialism".
Other Momentum candidates, Yasmine Dar and Rachel Garnham, were voted in while comedian Eddie Izzard lost out.
As Labour's supreme decision-making committee, the NEC plays a key role in the overall direction of the party as well as helping set the rules for leadership contests.
Mr Corbyn has gradually extended his influence over the NEC since he was first elected leader in September 2015, with more candidates sympathetic to his policies being elected to the 39-member body.
At last year's party conference, members agreed to create three new positions for party members on top of the six that already exist.
As well as Mr Corbyn and his deputy Tom Watson, there are six other MPs elected by their colleagues to the NEC - John Trickett, Kate Osamor, Rebecca Long-Bailey, George Howarth, Margaret Beckett and Shabana Mahmood.
Others on the body include 15 trade union representatives, two councillors, two members of socialist societies and representatives of Scottish Labour, Welsh Labour, MEPs in the European Parliament and Young Labour.
Mr Lansman is a key ally of the party leader who has been a leading figure on Labour's "hard left" for four decades.
He hailed his election as a "victory for 21st Century socialism".
Really honoured to now represent almost 600,000 members on the national executive of @UKLabour - at last the 21st century version of the Socialist party I joined 44 years ago pic.twitter.com/53ot5DMpjO— Jon Lansman (@jonlansman) January 15, 2018
Who sits on Labour's NEC
- Jeremy Corbyn (leader)
- Tom Watson (deputy leader)
- Andy Kerr (chair)
- Jenny Formby (vice chair)
- Diana Holland (treasurer)
- Six MPs
- One member of Scottish Labour (Richard Leonard)
- One member of Welsh labour (Alun Davies)
- Leader of Labour MEPs (Richard Corbett)
- Young Labour representative (Jasmin Beckett)
- 15 trade unionists (including Kerr and Formby)
- Nine representatives of constituency Labour parties
- Two councillors
On Sunday, the 60-year-old told the BBC 5 live's Pienaar's Politics his election would bring the "dream of a members-led Labour Party" a step closer.
"I think, I hope, that's what we'll have," Mr Lansman said. "Members will have nine out of a 39-member executive, still under a quarter, but much better representation, a reward for the 600,000 members who achieved such a fantastic turnaround in the general election."
Yasmine Dar is a Manchester councillor while Rachel Garnham has been a longstanding member of Labour's National Policy Forum - both are also members of Momentum.
Constituency branches were able to nominate three eligible candidates each. Anyone who had been a Labour member since 19 November was able to cast a vote.
Mr Izzard, regarded as being on the "soft left" of the party, had insisted he wants to work with those from other factions of the party to win the next general election after June's better-than-expected performance consolidated Mr Corbyn's position as leader.
After coming fourth, he congratulated the three winners and said he would continue to campaign for an "open and welcoming" vision of what he believed Labour should represent.
Labour MP Emma Reynolds, a critic of Mr Corbyn in the past, said it remained to be seen what impact the election would have on the balance of power within the NEC.
But she told the BBC's Daily Politics there was a "unity of purpose" within the party that had not existed in the early months of Mr Corbyn's leadership.