Guilmet attended the University of Arizona from 2006 to 2009. In 2007, he was the Pac-10 Pitcher of the Year. He was drafted by the Oakland Athletics in the 22nd round of the 2008 Major League Baseball Draft, but did not sign and returned to Arizona.
Eight days after the Indians designated him for assignment on March 30, 2014, Guilmet was traded to the Baltimore Orioles for Torsten Boss on April 7. He spent most of the season with the AAA Norfolk Tides where he appeared in 40 games. After pitching six scoreless innings in his first five games with the Orioles, he picked up his only decision with the ballclub in a 3-1 loss to the Houston Astros at Minute Maid Park on May 29. He made a total of 10 relief appearances with the Orioles, surrendering six earned runs and eight hits in 10 innings. He was designated for assignment on September 27 when the Orioles needed to make room on its 40-man roster for Alexi Casilla who had been promoted from the Tides.
Toronto Blue Jays
Guilmet was acquired by the Pittsburgh Pirates for cash considerations on October 3, 2014. He was designated for assignment on December 19, and subsequently claimed off waivers by the Toronto Blue Jays on December 23. Guilmet was optioned to the Triple-A Buffalo Bisons on March 15, 2015. On May 10, he was designated for assignment by the Blue Jays.
Guilmet was claimed off waivers by the Tampa Bay Rays on May 13, 2015, and optioned to the Triple-A Durham Bulls. After 4 games with Durham, he was recalled by the Rays on May 24, and optioned back to Durham after that day's game. He was designated for assignment on July 7.
Guilmet was claimed off waivers by the Los Angeles Dodgers on July 10, 2015, and optioned to the Triple-A Oklahoma City Dodgers. The Dodgers designated him for assignment on July 21.
Milwaukee Brewers
The Milwaukee Brewers claimed Guilmet off waivers on July 31, 2015. He was outrighted to the Triple-A Colorado Springs Sky Sox on September 8. During the 2015 season, Guilmet played for four different Triple-A organizations, Buffalo, Durham, Oklahoma City and Colorado Springs. He made 39 appearances in the minors and had a 2–2 record with a 2.15 ERA and 46 strikeouts. He also pitched in the majors for Tampa Bay and Milwaukee, compiling a 0–0 record with an 11.05 ERA and six strikeouts in five outings.
He played with the Tokyo Yakult Swallows for the 2017 season.
St. Louis Cardinals
On January 5, 2018, the Cardinals signed Guilmet to a minor league deal with an invitation to Spring Training. He was assigned to the Triple-A Memphis Redbirds. St. Louis purchased his contract from Memphis on June 5, 2018, and he made his Cardinals debut the same night at Busch Stadium. He was designated for assignment on June 7. In 21 games with Memphis he was 0-0 with a 0.93 ERA and with a 35/5 K/BB ratio. In 2 games with the Cardinals he was 0-1 with a 22.50 ERA and with 3 strikeouts and no walks.
Toronto Blue Jays (second stint)
On June 9, 2018, the Toronto Blue Jays claimed Guilmet off waivers from the St. Louis Cardinals, and designated pitcher Deck McGuire for assignment. Guilmet joined the team in Tampa on June 11. He was designated for assignment on July 4, and outrighted to the Buffalo Bisons on July 7. Guilmet elected free agency on October 10, 2018. In 6 games with the Blue Jays he was 0-0 with a 9.00 ERA and with 5 strikeouts and 4 walks. In 3 games with Buffalo he was 0-0 with a 5.79 ERA and with 2 strikeouts and 1 walk.
On November 20, 2018, Guilmet signed a minor league deal with the Minnesota Twins. He did not make the team and was assigned to AAA Rochester Red Wings to start the season. He became a free agent following the 2019 season.
Miami Marlins
On February 5, 2020, Guilmet signed a minor league deal with the Miami Marlins.
