After winning the first test 23-21 and losing the second 16-15, the Lions didn't want any last-minute nerves, so they scored in the second minute of the series-decider and piled on more points in a test than they ever had in their 125 years of touring.
A week after missing a stoppage-time penalty that could have clinched the series for the Lions in Melbourne, Halfpenny shredded the Wallabies defence on the left side as he set up the decisive tries for Jonathan Sexton and George North in the 57th and 64th minutes. He landed five penalties and three conversions, and his 21-point tally for the match gave him 49 for the series — both Lions records.
"It's just a huge honour. But it's been a squad effort, and pretty special as a player," Halfpenny said after being voted the player of the match and the series. "It just all came together tonight."
Lions coach Warren Gatland gambled by making six changes for the deciding test, relying on 10 players from his Six Nations-winning Welsh team and dropping Irish veteran Brian O'Driscoll.
Gatland said he had to use his head and not his heart in the selections. He said he was stunned by the vitriolic criticism, but in the end was vindicated. After playing in the first two tests, O'Driscoll now has won his first series with the Lions — on his fourth tour.
"These guys have done themselves proud, have done that jersey proud and they deserve a huge amount of credit for that performance and what they've achieved," Gatland said. "The thing with test much rugby, it's agony or ecstasy. There's nothing in between. When you get the ecstasy, you've got to enjoy that moment. Not just for myself, but for everyone involved. Emotionally, we've put so much into it."
The Lions started at a frenetic pace, needing less than 90 seconds to open the scoring. English prop Alex Corbisiero rolled over adjacent to the posts three phases after the opening scrum — after Wallabies scrumhalf Will Genia knocked-on from the kickoff.
Australia coach Robbie Deans, who now faces the real prospect of losing his job, lamented the "horrific start" and a terrible scrum performance.
"They used their set-piece with great advantage and then fed off it," he said. "It worked for them, they were turning the scoreboard over and you saw the rest.
"We gave them a leg-up, which really allowed them to thrive."
The match was divided into three distinct periods: the Lions raced to a 19-3 lead within 25 minutes; the Wallabies rallied to get within three points at 19-16 soon after halftime; and the Lions responded by adding 22 points, including three tries, in the last 28 minutes.
It took less than 90 seconds to set the tone through Corbisiero's try. Halfpenny converted and added four more first-half penalties to earn his individual Lions record — surpassing Neil Jenkins' 41 points in the 1997 series — as the Lions grabbed a 16-point lead.
Halfpenny was able to accumulate points regularly because the big Lions pack was smashing the Australians at scrum time, earning two free kicks and two penalties from the first five scrums before Wallabies prop Ben Alexander was sin-binned for another collapse that lead to the fourth penalty goal.
"We had too many errors, we gave them a head start," Australia captain James Horwill said. "You have to give them credit, they were the better team tonight. We let ourselves down in the scrum."
Alexander was replaced by Sekope Kepu after the yellow card, meaning openside flanker George Smith also had to leave the field for 10 minutes to cater for the frontrow change and leaving only seven men in the Wallabies scrum.
It was the second stint on the sidelines for Smith, playing his first test in almost four years, after he needed a five-minute medical assessment following a sickening clash of heads with Lions hooker Richard Hibbard in the fifth minute.
The momentum was heavily against the Australians and it was compounded when blockbusting winger Israel Folau limped off in the 27th minute to be replaced by new cap Jesse Mogg, leaving only one specialist back on the Wallabies bench.
Despite the numeric advantage, the Lions weren't able to add to their cushion and missed a chance to go further ahead when Sexton's attempted dropped goal after 27 phases of grafting forward movement just drifted to the left of the uprights.
The Wallabies gradually got back into the contest as their scrum steadied, eventually setting the platform for James O'Connor's close-range try just before halftime.
Two Christian Leali'ifano penalty goals after the break reduced the margin to three points, but then the Lions seized the momentum.
After conceding 13 straight points, the Lions finally got back on the scoreboard with Halfpenny kicking his fifth penalty in the 51st minute and then went further ahead when the fullback made a break on the left side and popped an inside ball for Sexton to score untouched in the 57th. Seven minutes later, he sent North into the left corner after another strong run.
Jamie Roberts, recalled after missing the first two tests with a hamstring injury, crossed three minutes later to put the result beyond any doubt.
Lions supporters, making up almost half the 83,702 crowd at Sydney's Olympic stadium, started singing "Swing Low" as the Welsh centre crossed and kept up the chorus until the end.
The Lions last won a series in South Africa in 1997. They won the first test against Australia in 2001 but lost the next two to surrender a series to the Wallabies. They lost 3-0 in New Zealand in 2005 and 2-1 in South Africa in 2009, adding to the pressure on Gatland's team to end the drought.
The emphatic ending of the drought puts any doubt about the future of the popular Lions concept to bed.
British and Irish Lions 41 (Alex Corbisiero, Jonathan Sexton, George North, Jamie Roberts tries; Leigh Halfpenny 3 conversions, 5 penalties), Australia 16 (James O'Connor try; Christian Leali'ifano conversion, 3 penalties). HT: 19-10. Attendance: 83,702.