Junior and amateur career Born in
Vizzolo Predabissi, Guarnieri finished in the top ten in the road race at both the
UCI Junior World Championships (fourth) and the
UEC European Junior Road Championships (eighth) in 2005. He also finished third in the
Driedaagse van Axel, and won the points classification at the race. Ageing out of the juniors for 2006, Guarnieri joined the team, where he took several successes in 2007 – winning the
Circuito del Porto and
Trofeo Alcide De Gasperi one-day races, as well as a stage at the
Olympia's Tour. The following year, he won the
ZLM Tour – held as part of the
UCI Under 23 Nations' Cup – and the opening stage of the
Giro delle Regioni, which earned him a contract as a
stagiaire with towards the end of 2008.
Liquigas (2009–2011) Guarnieri signed permanently with for the 2009 season, Later in the season, he took his first professional victory when he won the third stage of the
Tour de Pologne into
Lublin, beating
Allan Davis and
André Greipel in the sprint. In 2010, he recorded another early season second-placed finish, to
Roberto Ferrari at the
Giro del Friuli in March, before taking another stage victory at the
Tour de Pologne, winning the opening stage in
Warsaw to take the race lead. He made his
Grand Tour début at the
Vuelta a España, before finishing fourth overall at the
Circuit Franco-Belge, winning the second stage into
Poperinge as part of a 1–2–3 for . He took one further victory with the team in 2011, when he won stage 3a of the
Three Days of De Panne.
Astana (2012–2014) Guarnieri transferred to for the 2012 season, having signed an initial two-year contract with the team. In his first season, Guarnieri's best individual result was a second-place stage finish on the opening stage of the
Three Days of De Panne – beaten only by his former teammate
Peter Sagan – while he also finished in fourth place at the
Grote Prijs Stad Zottegem one-day race. Despite no top-five results in 2013, Guarnieri remained with the team into the 2014 season, but suffered a fractured
fibula early in the year at
Tirreno–Adriatico. He missed two months of racing as a result, and his best finish thereafter, was second place on stages at the
Tour of Austria and the
Tour de Pologne.
Team Katusha (2015–2016) In September 2014 it was announced that Guarnieri would join from 2015 on a two-year contract, with the team's general manager
Viatcheslav Ekimov emphasising Guarnieri's role as part of the
sprint train, as a lead-out man, for
Alexander Kristoff. Guarnieri supported the majority of Kristoff's twenty victories during his first season at the team, but they went winless at the
Tour de France where Guarnieri made his début. Guarnieri's best individual result of the year was a second-place stage finish at the
Eneco Tour, losing out to
André Greipel in a sprint finish. Guarnieri supported a further eight victories for Kristoff in 2016, with one such instance at the
Tour of Qatar resulting in Guarnieri taking third on the stage.
FDJ (2017–2022) After two seasons with , Guarnieri joined for the 2017 season on an initial two-year contract, forming part of the
sprint train for the team's main sprinter
Arnaud Démare. On their first stage working together, Démare won the opening stage of
Étoile de Bessèges in
Beaucaire. Guarnieri formed part of a further seven victories for Démare, including stage wins at
Paris–Nice, the
Critérium du Dauphiné and the
Tour de France. At the Tour de France, he was involved in an incident with
Nacer Bouhanni, with Guarnieri apologising for comments aimed at Bouhanni. Guarnieri assisted a further nine wins for Démare in 2018, including another
Tour de France stage victory, where Guarnieri had moved Démare to the front on the run-in to the finish in
Pau. He extended his contract with the team for a further two years in August 2018, until the end of the 2020 season. Guarnieri made his début at the
2019 Giro d'Italia, as part of the sprint train for Démare, with Démare winning the tenth stage. They returned to the race
the following year, held in October due to the
COVID-19 pandemic, with Démare having taken nine victories to that point in the season. In the first half of the race, had prioritised four sprint stages for Démare – and won them all with lead-outs from Guarnieri and
Ignatas Konovalovas, with Guarnieri later stating that the team had ridden the "perfect Giro d'Italia", as Démare won the
points classification. He also signed a further two-year contract extension with the team, until the end of the 2022 season. Guarnieri had formed part of all Démare's victories in the 2021 season, prior to the
Tour de France, with the latter targeting sprint victories at the race. Guarnieri and Démare both finished outside the time limit after the ninth stage, and were eliminated. The duo rode the
2022 Giro d'Italia, with Démare winning three stages and the points classification at the race.
Lotto–Dstny (2023–2024) After six seasons with , Guarnieri signed a two-year contract with – later renamed as – from the 2023 season, signing to be a member of the
sprint train for the team's sprinters such as
Caleb Ewan and
Arnaud De Lie. He was part of the sprint train that led Ewan to victory at the 2023
Van Merksteijn Fences Classic, and was selected for
that year's Tour de France, but had to abandon the race with broken ribs after a crash in the final sprint of stage four at the
Circuit Paul Armagnac in
Nogaro. In September 2024, he announced his retirement from the sport. ==Personal life==