Marchi rode the first few years of his professional career in Brazil before moving to the United States. He competed very briefly in the
Championship Bull Riding (CBR) tour in 2004 before joining the PBR full-time. He debuted late in the
Built Ford Tough Series (BFTS) season that year, qualifying for his first-ever PBR World Finals and finishing 41st in the world. After finishing in the runner-up position for the PBR World Championship in three consecutive years, he won his world title in 2008. Statistically, Marchi was one of the most consistent riders on the tour. He would go on to qualify for the World Finals all 15 years of his PBR career (2004 through 2018). In 2005, Guilherme Marchi was the biggest threat to
Justin McBride's goal of being the PBR World Champion. McBride won, Marchi came in second. However, Marchi was the PBR World Finals event champion that year. In 2006, during one of the last few regular-season BFTS events before the World Finals, Marchi won the first round of the event in
Greensboro, North Carolina, and became eligible for the
Mossy Oak Shootout. He successfully rode his bull and won the $90,000 bounty. At the World Finals, Marchi once again failed to win when fellow Brazilian
Adriano Moraes came from behind to claim the PBR world championship, with Marchi placing second. Yet again, in 2007, Marchi finished the season in the No. 2 position behind Justin McBride at the PBR World Finals in
Las Vegas. In 2008, Marchi dominated the PBR circuit, riding nearly 75% of his bulls, winning five events, and earning over $1.5 million (nearly three times as much as any other PBR rider) on his way to his PBR world championship title and the $1 million bonus that went with it. In 2009, Marchi won the PBR Canada Finals event in
Calgary, Alberta. In March 2014, in the opening round of the BFTS event in
Phoenix, Arizona, Marchi became the first bull rider to successfully complete 500 qualified rides in the PBR, riding Pandora's Pyxis. On April 8, 2017, Marchi rode Shocker for 86.25 points in Round 2 of the BFTS event in
Billings, Montana. In doing so, he became the first PBR rider to reach 600 qualified rides and received a commemorative belt buckle. On August 12, 2018, Marchi announced that 2018 would be his final year of bull riding, with the intention of retiring after the PBR World Finals. He finished his professional career with 635 qualified rides (the most in PBR history), 36 PBR event wins, 15 consecutive qualifications to the PBR World Finals (2004 through 2018), and over $5.3 million in career earnings. Marchi officially retired from bull riding following his victory at the 2018 PBR Brazil event in
Goiânia,
Goiás. ==Post-career==