Phinney was born on June 27, 1990 to former professional road cyclist and Olympic medal-winner Davis Phinney and former Olympic gold medal-winning cyclist and speed skater Connie Carpenter-Phinney.

In 2007 at the age of 16, Phinney began racing on Team Slipstream’s junior squad. Slipstream team manager Jonathan Vaughters signed Phinney to the team before he had competed in a race, having heard word-of-mouth reports about Phinney’s ability on group rides in Boulder. It was at this time that Phinney was introduced to track cycling. In August 2007, he won the World Junior Championships time trial title. Since then, Phinney has competed in National, World Cup and World Championship events for track cycling. Phinney finished seventh in the individual pursuit at the 2008 Summer Olympics. Later that year, at the U.S. National Track Championships, he won gold medals in the elite one kilometer time trial, individual pursuit and team pursuit races.

On September 24, 2008, Lance Armstrong announced that Phinney had made the under-23 team, which was organized by the group that managed Armstrong’s Team RadioShack, Trek–Livestrong. On March 26, 2009, Phinney won the individual pursuit at the 2009 UCI Track Cycling World Championships and, again, at the 2010 UCI Track Cycling World Championships on March 25, 2010. Phinney then switched his focus to the road, winning the Paris–Roubaix Espoirs, the Olympia’s Tour overall, in addition to the first four stages and time trials in Tour de l’Avenir and Tour of Utah.

On July 29, 2010, it was announced that Phinney and teammates Jesse Sergent and Clinton Avery would ride in the Tour of Denmark for Team RadioShack, riding as stagiaires.

On September 22, 2010, the BMC Racing Team announced that Phinney would become part of BMC in 2011, joining a team that included Cadel Evans, George Hincapie and Alessandro Ballan. The highlight of Phinney’s first season with BMC was a fourth-place finish in the Eneco Tour.

Phinney spent the early part of the 2016 season recovering from his injuries, making his first race appearance of the year at the Tour du Haut Var in February and subsequently racing in the spring classics before returning from Europe to his native Colorado. In May 2016 Phinney took his third national time trial title in Winston-Salem, North Carolina, beating runner-up Tom Zirbel by over a minute: Phinney expressed satisfaction with the result and his performance, hoping that it would help him secure selection for the 2016 Summer Olympics. He competed at the Olympics, but despite entering the road time trial with hopes of a medal, he finished 22nd, over five minutes behind winner Fabian Cancellara.

In September 2016 Phinney confirmed that he had agreed an initial two-year deal with Cannondale–Drapac from the 2017 season, with a focus on competing in the classics and aiming to race in the 2017 Tour de France. In June 2017, he was named in the start list for the 2017 Tour de France, marking his debut in the race. In an interview ahead of the Tour Phinney stated that he was still undergoing therapy to deal with the effects of his injuries, and said that the power output from his left side was almost 25 percent down on that from his right side when making an explosive effort. Phinney was leading the mountains classification and wearing the polka dot jersey competition on stage 2.

In October 2019 Phinney announced that he would retire from professional racing at the end of the 2019 season. He cited the ongoing effects of an injury he suffered in 2014 as a reason for his retirement.