John Thomson's journey through Major League Baseball began when he was drafted by the Colorado Rockies in the seventh round of the 1993 MLB Draft. A right-handed pitcher, Thomson made his major league debut with the Rockies on May 11, 1997. During his time with Colorado, he developed a reputation for his solid pitching mechanics and ability to handle the pressures of Coors Field, a notoriously difficult park for pitchers due to its high altitude.

Thomson spent several seasons with the Rockies, where he honed his skills and gained valuable experience. In 2000, he was traded to the New York Mets, but his time there was short-lived. He was soon on the move again, this time to the Texas Rangers in 2001. With the Rangers, Thomson found more stability and became a regular in their starting rotation. His tenure with Texas allowed him to further refine his pitching style, and he delivered several commendable performances during his stay.

In 2003, Thomson signed with the Atlanta Braves, a team known for its strong pitching staff. Joining the Braves provided him with the opportunity to work alongside some of the best pitchers in the league. During the 2004 season, Thomson posted a career-best 14-8 record with a 3.72 ERA, helping the Braves secure another division title. His performance that year was a highlight of his career, showcasing his ability to compete at a high level.

After his stint with the Braves, Thomson continued his career with the Kansas City Royals in 2006. However, injuries began to take a toll on his performance, limiting his effectiveness on the mound. Despite these challenges, he persevered and continued to contribute to his teams as best as he could.

Thomson's final major league appearance came with the Toronto Blue Jays in 2007. Although his time in Toronto was brief, it marked the end of a decade-long career in the majors. Over the course of his career, Thomson played for several teams, adapting to different environments and overcoming various challenges along the way.

Throughout his career, Thomson was known for his professionalism and work ethic. He faced the ups and downs of a professional athlete's life with determination, earning the respect of teammates and opponents alike. While injuries ultimately shortened his time in the majors, his perseverance and dedication to the game were evident throughout his career.