Talmácsi began racing on minibikes built by his father at the age of four, and also competed in boxing as a youth. He won the 1996 Hungarian 125cc national championship and progressed through the European championship circuit, finishing fifth in the European 125cc Championship in 2000. That year, competing for a German team, he produced enough results to attract sponsorship for Grand Prix racing from 2001 onwards. He has a younger brother, Gergő, who is also a motorcycle racer.
Talmácsi competed in the 125cc World Championship from 2001, working his way up through a succession of teams. Early seasons with Racing Service, Italjet, the PEV ADAC Sachsen team, Exalt Cycle Aprilia, and Malaguti yielded steady but unspectacular results. His best finish with Malaguti in 2004 was seventh place at Estoril on non-competitive machinery, which nonetheless attracted the attention of leading teams.
Joining the Red Bull KTM team for 2005 brought Talmácsi his first competitive bike. He scored his maiden 125cc podium at Shanghai in the third race of the season, and inherited his first victory at Mugello when both Mika Kallio and Héctor Faubel fell on the final lap. He won again at Assen and claimed a third win in Qatar.
The Qatar race became the most controversial of Talmácsi's career. With four rounds remaining, his teammate Kallio still had a realistic chance of defeating Thomas Lüthi for the title, while Talmácsi held only a mathematical outside chance. The KTM team instructed Talmácsi to ride behind Kallio and not contest the win, to protect Kallio's points. Talmácsi followed his teammate throughout the race, but on the final lap's finishing straight he pulled out of Kallio's slipstream and passed him in the closing meters, winning by 0.017 seconds. At the post-race press conference Talmácsi said he believed there was still one more lap to go, despite the team having shown them a board before the final lap. Kallio subsequently finished fifth in Australia, retired in Turkey, and entered the Valencia finale 23 points behind Lüthi — the exact difference in championship points between first and second place. Kallio won the Valencia finale but Lüthi scored enough to take the title by five points: the precise margin Talmácsi had taken from Kallio in Qatar. Kallio had also won four races but would have claimed five, and thus the title on countback, without the Qatar overtake. Talmácsi was not offered a new contract by KTM following the season finale.
For 2006, Talmácsi signed with the Humangest Honda team. The Japanese manufacturer had not significantly developed their 125cc machine following Lüthi's championship, leaving Aprilia with a substantial competitive advantage. Álvaro Bautista won the title for Aspar Aprilia by a wide margin, with Talmácsi's best result a third place at Brno. He finished seventh overall with 119 points, the top Honda finisher.
For his seventh season in the 125cc class, Talmácsi joined Jorge Martínez's Aspar team, replacing outgoing champion Bautista. Riding the older RSW Aprilia engine while his Spanish teammates Héctor Faubel and Sergio Gadea used the newer but less reliable RSA engine, Talmácsi began the season with a second place at Qatar and then won at Jerez, overtaking Lukáš Pešek before the finish line.
His championship lead fluctuated throughout a tense season. He did not finish only one race, at Donington due to engine failure. A dominant win at the Sachsenring from pole position with the fastest lap secured his championship lead, though a fourth place at Brno — despite an estimated 30,000 Hungarian supporters making the trip to the nearest Grand Prix to their home country — ceded momentum to rivals.
The title battle tightened to a single-point margin going into Australia, where Talmácsi finished eighth. At Sepang he led from the first lap to win convincingly, arriving at the Valencia finale with a ten-point advantage over Faubel. In a tense final race, Faubel tried numerous tactics including rallying a fellow Aspar rider against Talmácsi, but the Hungarian held second place throughout. Faubel won the race, but Talmácsi's second place secured the championship by one point. He became the 2007 125cc World Champion, the first world champion from Hungary, and the first from the central-eastern European region. The celebrations in Budapest were nicknamed "Talmageddon" — a portmanteau of his surname and Armageddon — and he was named 2007 Hungarian Sportsman of the Year.
Talmácsi remained at Aspar for 2008 with an RSA Aprilia, producing his most consistent season with three wins at Assen, Misano, and Sepang, five third places, and one second place. He finished third in the championship with 206 points.
Talmácsi moved to the 250cc class with Aspar's Balatonring Team for 2009, posting a fourth place in Japan before a contractual dispute led to his departure after only three races. With backing from Hungarian oil company MOL Group, he joined Scot Racing in MotoGP from the sixth round onwards. He scored his first MotoGP point in Germany and collected 19 points over eight consecutive race finishes, with a best result of 12th at Donington ahead of notable names in wet conditions.
For 2010 Talmácsi contested the newly formed Moto2 class with Speed Up alongside Andrea Iannone, finishing sixth in the championship with 109 points and one podium, a third place at Aragón. He was unable to secure a ride for the 2011 season.
In 2013 Talmácsi returned to racing in the Supersport World Championship, but suffered a catastrophic leg injury at the Portimão Circuit when an engine failure caused a metal fragment to strike his shin with sufficient force to break his tibia and destroy a significant portion of the anterior muscle in his leg. The injury ended his racing career.
Following his retirement, Talmácsi established Talmácsi Racing, which competed in the European Superstock 600 Championship with Honda CBR600RR machinery from 2014 until the series concluded after the 2015 season.
Talmácsi is the sole Hungarian to have won a motorcycle road racing World Championship. His 2007 125cc title, claimed on the final lap of the final round of the season, stands among the most dramatic championship conclusions in the class's history. The 2005 Qatar incident remains deeply linked to his name — an act that reshaped the 125cc championship, cost his teammate the title, and ended his relationship with KTM, yet also demonstrated the decisive opportunism that would define his best racing moments.