Letarte began working for Hendrick Motorsports part-time in 1995. In 1996, at the age of sixteen, he joined the team full-time. From 1997 to 1999, he served as a tire specialist for Jeff Gordon's No. 24 team before advancing through the roles of mechanic and then car chief. His rapid progression reflected the depth of trust the organization placed in him from an early age.
Letarte was promoted to crew chief for Gordon midway through the 2005 season, following Gordon's failure to reach the Chase for the NEXTEL Cup. In his sixth race in the role, he led Gordon to victory at the Subway 500 at Martinsville Speedway in October 2005, his first career Cup Series win as a crew chief.
The partnership's strongest season came in 2007, when Gordon recorded six wins — his highest total since 2001 — along with a series-leading 21 top-five finishes and 30 top-ten finishes, setting a NASCAR modern-era record for most top tens in a season. Despite a dominant points season and strong Chase performance, Gordon lost the championship to Jimmie Johnson, who had a superior win count across the playoff races.
Gordon went winless in 2008, a difficult year that prompted outside criticism of Letarte's leadership, but team owner Rick Hendrick and Gordon publicly supported their crew chief. The decision proved justified: in 2009, Gordon snapped a career-high 47-race winless streak with a victory at Texas Motor Speedway, and the No. 24 team finished third in the season standings with 16 top-five finishes and 25 top tens.
For the 2011 season, Hendrick Motorsports reorganized its crew chief assignments. Letarte was reassigned from Gordon to Dale Earnhardt Jr.'s No. 88 car. Earnhardt, who had been through a difficult stretch of winless racing and inconsistent results, responded quickly under the new partnership. Letarte guided him back into the Chase in 2011, the first time Earnhardt had qualified for the playoff since 2008.
Earnhardt's 143-race winless streak ended in June 2012 with a victory at Michigan International Speedway — a win Letarte helped engineer with strategic pit calls. The pair's most celebrated achievement came on February 23, 2014, when Earnhardt won the Daytona 500 in a narrow finish over Denny Hamlin and Brad Keselowski. The season also included two wins at Pocono Raceway and a victory in the Goody's Headache Relief Shot 500 at Martinsville Speedway — Earnhardt's first Martinsville clock. It was the first time Earnhardt had recorded multiple wins in a season since 2004.
On January 9, 2014, it was announced that Letarte would leave Hendrick Motorsports after the 2014 season to join NBC Sports as a NASCAR analyst. His first broadcast was the 2015 Coke Zero 400 at Daytona, won by Earnhardt — reuniting driver and departed crew chief in a different context. Letarte spent years as part of NBC's NASCAR coverage, building a reputation as an articulate and technically knowledgeable voice.
In November 2024, it was announced that Letarte would join Amazon Prime Video and TNT's NASCAR coverage as a color commentator beginning with the 2025 television contract cycle, while continuing his work for NBC. The arrangement reunited him with Earnhardt, who had also become a broadcaster with NBC.
Letarte also served as a consultant for Spire Motorsports beginning in December 2020, briefly returning to the pit box in February 2021 for a single race as interim crew chief when Spire's regular crew chief was unavailable due to COVID-19 protocols.
Letarte is recognized as one of the more thoughtful and technically accomplished crew chiefs of his era at Hendrick Motorsports. His ability to rebuild Jeff Gordon's competitiveness mid-career and then extract multiple wins from Dale Earnhardt Jr. — a driver whose results had been inconsistent for several seasons — demonstrated consistent adaptability across different driver personalities and car setups. His subsequent broadcast career has made him one of the more prominent voices in American motorsport commentary.