Former Olympic 1500m champion Lord Coe is the new president of the IAAF, the body that governs world athletics.
The 58-year-old Briton beat rival Sergey Bubka, a former Olympic pole vault champion, by 115 votes to 92.
One of Coe's first tasks will be to deal with the fall-out from a series of doping allegations to hit athletics.
Double Olympic champion Mo Farah and Olympic 1500m silver medallist Steve Cram have backed their countryman to make a positive impact.
Following his election, Coe tweeted that, after the birth of his children, this was the most "momentous moment" of his life.
The former chairman of London 2012, Coe replaces 82-year-old Senegalese Lamine Diack, who has been in charge for 16 years.
Who is Lord Coe?
One of Britain's finest sportsmen, having won gold medals in the 1500m at the 1980 and 1984 Olympics and set numerous world records over both the 1500m and 800m.
His battles and rivalry with fellow Brits Steve Ovett and Cram during the 1980s became legendary and increased the popularity of middle-distance running enormously.
Coe has also been a Conservative MP, worked for Fifa and was a key figure in London's successful bid for and staging of the 2012 Olympics and Paralympics.
He was appointed an MBE in 1982, an OBE in 1990 and accepted a life peerage in 2000.
An IAAF vice-president since 2007, he becomes only the sixth president in the organisation's 103-year history.
What's his reaction to winning?
He seemed very happy, as his initial tweets indicated.
In a subsequent interview with BBC sports editor Dan Roan, he said his appointment was "a big challenge but a great honour".
Coe added: "My sport is what fires and drives me.
"I am fortunate to be one of those people who has woken up - since the age of 11 - and athletics has shaped my day. I want to do everything I can to make sure it is in the best possible shape."
Beaten opponent Bubka, 51, who was re-elected as a vice-president, said: "I continue to serve athletics with dignity and big passion."
What do others think of Coe's election?
From a domestic point of view, reaction has been very positive.
Cram, who won silver behind Coe at the 1984 Olympics in Los Angeles, told BBC Radio 5 live: "It's a great result for the sport."
He added Coe was the "right guy" to lead the IAAF, praising his leadership qualities and the respect he commands.
"It's not going to be easy," added Cram, "but I think Seb's prepared to be tough himself to hopefully make those changes."
UK Athletics chairman Ed Warner said Coe's election was "momentous for the sport" .
He added: "This felt like a crossroads for me. The challenges facing athletics are such that we need a great president right now and I do believe Seb's got the range of skills and the character that's required to really pull us forward."
But he warned against a quick solution to the drug issues, insisting: "It's not an overnight success story that Seb is going to ring here."
Warner also said athletics needed Coe's "commercial nous" at a time when the prestigious Diamond League, an annual series of track and field meetings held around the world, was without a headline sponsor.
What about global reaction?
USA Track and Field said it had been "impressed" with Coe's "vision for the sport", "his dedication to its cause" and "his demonstrated effectiveness as a leader".
European Athletics president Svein Arne Hansen said he was looking forward to working closely with Coe "for the good of our sport".
That sentiment was echoed by World Anti-Doping Agency president Craig Reedie, who referenced Coe's "avowed plans to protect the rights of the clean athlete".
Athletics Australia president David Grace said Coe's "wealth of knowledge in the field of sports governance and administration" will "ensure that athletics globally is in capable hands in years to come".
Outgoing president Diack added: "For me, it's a dream come true that I can pass on the baton to Sebastian, who has been prepared for the job."
What's Coe's main task?
Helping restore the battered image of athletics following a number of harmful doping cases and allegations.
Earlier this month, The Sunday Times and German broadcaster ARD/WDR alleged an "extraordinary extent of cheating" after obtaining blood test data from 5,000 athletes between 2001 and 2012.
Coe, who will hold an initial term of four years, claimed the reports were a "declaration of war" on his sport.
He has already promised to create of an independent anti-doping panel to address the issue within his first 100 days in office.
Warner added: "If there's one person that I know will pursue cheats to all four corners of the earth, it is Seb."
How significant is Coe's election?
Coe's tenure will be ultimately judged on how successful he is in the fight against doping, according to BBC Radio 5 live sports news correspondent Richard Conway.
"As Warner has said, the IAAF must now do the right things and be seen to do the right things," added Conway.
"That is Lord Coe's challenge in both the short and long term.
"But if you want a signal of the scale of the problems he faces over public confidence, then look no further than the men's 100m final at the World Championships in Beijing on Sunday.
"If Justin Gatlin, a man convicted not once but twice for doping offences, beats Usain Bolt, the sport's poster boy, it will merely underline the task ahead for athletics and for Coe."
What else does Coe plan to do?
Tackling the drugs issues aside, he has also pledged to:
- Revitalise the athletics calendar;
- Increase youth participation;
- Strengthen national federations;
- Increase revenue.
Coe has highlighted the need for a more structured calendar and the need to be more innovative in how the IAAF presents the sport.
In his manifesto, he said he wants to consider an 'IAAF Street Athletics' circuit in major cities across the world.
He also wants to increase the quality on show at the Diamond League events, the IAAF's flagship one-day meetings.
As far as the national federations are concerned, Coe intends to give an Olympic Athletics Dividend, which would provide at least US$ 100,000 of extra funding over four years, to all 214 IAAF member federations.