Following his win at the 2017 UCI World Road Race Championships, Sagan wore the rainbow jersey for the duration of 2018 for an unprecedented third consecutive season.

[15] Sagan started to ride bikes at the age of nine when he joined Cyklistický spolok Žilina, a small local club in his home town.

[20] Sagan was focused on continuing his career as a mountain bike rider, but his management company Optimus Agency approached several professional road cycling teams.

In November 2009, Liquigas' Stefano Zanatta, Paolo Slongo and Enrico Zanardo offered Sagan, who spoke neither Italian or English, a ten-month contract valued at €1,000 per month.

[25] Sagan won his two ProTour stages during Paris–Nice, a race he was not initially nominated for, but joined the team after his teammate Maciej Bodnar broke his collarbone.

[34] He planned to ride several late season races including Paris–Tours and the Giro di Lombardia but an intestinal issue forced him out of the former and weakened him in the latter.

Sagan's good form continued into the classics season, with fourth place in Milan–San Remo,[53] second in Gent–Wevelgem,[54] a stage victory in the Three Days of De Panne,[55] fifth in the Tour of Flanders,[56] and third in the Amstel Gold Race.

He kicked things off with a win in the opening prologue, besting local favourite and time trial specialist Fabian Cancellara (RadioShack–Nissan) by 4 seconds over the 7.3-kilometre (4.5-mile) course.

[66] With about 350 metres (1,150 feet) to go on the fourth stage, Marcus Burghardt of BMC Racing Team launched a sprint for the finish line in rainy conditions.

He won the first stage in Seraing atop a small climb after breaking away with a little more than a kilometre to go with Fabian Cancellara (RadioShack–Nissan) and out sprinting him and Edvald Boasson Hagen (Team Sky).

[71] As Sagan crossed the finish line, he performed a "running man" salute akin to the eponymous character portrayed by Tom Hanks in 1994 film Forrest Gump.

[72] He won again on Stage 6, which had a course suited for a bunch sprint; he beat pure sprinters André Greipel of Lotto–Belisol and Matthew Goss of the Orica–GreenEDGE squad by around a bike length in Metz.

[78] He took his first victory of the season on the second stage of the Tour of Oman, as he broke away from a chasing group in the final kilometres, joined and dropped three escapees before soloing to the win.

[84] On stage 6, Sagan survived a climb featuring a section at a 30% incline, and formed a breakaway with former teammate Vincenzo Nibali and Joaquim Rodríguez, beating both in the sprint.

Sagan broke away from a group of ten riders with 4 kilometres (2.5 miles) remaining and won solo, performing a series of wheelies after crossing the line.

[96] Sagan cemented his victory in the points classification by taking the eighth stage, which was flatter and more suited to the sprinters, ahead of Daniele Bennati and Gilbert.

Sagan fell short of success in the Grand Prix Cycliste de Québec where he accelerated on one of the final climbs but faded in sight of the finishing line.

Two days later, he took victory in the Grand Prix Cycliste de Montréal, escaping the leading group on a climb with 5 kilometres (3.1 miles) to go and winning solo.

[125] Sagan started his season at the Tour of Qatar, taking his first podium spots with second-place finishes on stages four,[126] and five,[127] and won the young rider classification.

[151] In the Tour de France, after he suffered a flat tyre and caught back up to the 25-rider lead group, Sagan took second position on Stage 2 behind André Greipel.

[168] In February, after a three-week training camp in Spain's Sierra Nevada, he competed in the opening races of the Belgian classics season, finishing second in Omloop Het Nieuwsblad and seventh in Kuurne–Brussels–Kuurne.

During the race, he avoided a crash by Fabian Cancellara directly in front of him, by bunny hopping over his bike, despite having only one foot clipped into his pedals at the time.

[181] He also won the next day, joining the late breakaway after attacking the reduced peloton at the end of a climb, and outsprinted Michael Albasini and Silvan Dillier.

He came into the finish with the other 24 breakaway riders, and won the subsequent bunch sprint ahead of Great Britain's Mark Cavendish and Tom Boonen of Belgium, respectively.

[202] After training for the next few weeks, Sagan finished in second place at Omloop Het Nieuwsblad to Olympic Road Race Champion Greg Van Avermaet.

With 16.9 kilometres (10.5 miles) to go and 59 seconds down, Sagan's handlebar was caught on a jacket draped over the spectator barrier on the Oude Kwaremont causing him to crash, and ruining his chances of victory.

[213] After the bunch sprint finish of stage four, in which Sagan placed second, he was disqualified after race officials judged that he caused Mark Cavendish to crash, with the jury president Philippe Marien saying that he "endangered some of his colleagues seriously".

[271] He won his eighth Slovak National Road Race Championships the following week,[272] finishing nearly two minutes clear of his closest competitor, Lukáš Kubiš.

[278] After retiring from the road to focus on qualifying for the 2024 Olympics mountain bike competition, Sagan was diagnosed with anomalous tachycardia, a cardiac arrhythmia and acute bradycardia[citation needed] in February 2024.

[279] Having missed the start of the mountain bike season, Sagan returned to road racing, signing for the UCI Continental team Pierre Baguette.