Tomko attended El Dorado High School in Placentia, California, and was a letter winner in basketball and baseball.
Tomko attended Florida Southern College for one season in 1995, leading the team to the NCAA Division II National Championship against Georgia College. He went 15-2 with a 1.35 ERA and struck out 154 batters in 126.3 innings that season, with opponents hitting just .180 against him. He pitched two complete game shutouts in the Championship Series, including one in the final game, earning him the Tournament's "outstanding player" award. In addition, he won both the NCAA Division II Pitcher and Player of the Year Awards by the American Baseball Coaches Association.
In 2014, the NCAA Division II Pitcher of the Year Award was renamed the Brett Tomko Award in his honor.
Tomko had been drafted out of high school by the Los Angeles Dodgers in the 20th round of the 1994 draft, but chose to attend college at Mt. San Antonio College (Walnut, California) for a year. In 1995, he was drafted in the second round by the Cincinnati Reds. He signed with the Reds on June 28, 1995.
He pitched for three years in the Reds minor league system, making stops at Charleston in 1995, Chattanooga in 1996 (where he was named the Reds' top prospect by Baseball America) and Indianapolis in 1997 before getting called up to the Major Leagues during the 1997 season.
Tomko made his first Major League appearance, and first Major League start, against the Philadelphia Phillies on May 27, 1997. He pitched 6 innings and gave up 2 runs while taking the loss in the Reds 2-1 defeat.
He got his first victory in his next start on June 6, against the New York Mets. He pitched 6 innings, gave up one run and struck out seven in the Reds 5-2 victory.
He remained in the Reds starting rotation for three seasons.
He was traded on February 10, 2000, to the Seattle Mariners, along with Antonio Pérez, Jake Meyer, and Mike Cameron for Ken Griffey, Jr.. Seattle used him primarily as a reliever and spot starter during the next two seasons. He also spent some time with Seattle's Triple-A team in Tacoma in both 2000 & 2001.
On December 11, 2001, the Mariners traded Tomko (along with Ramón Vázquez, Tom Lampkin and cash) to the San Diego Padres in exchange for Wascar Serrano, Alex Arias and Ben Davis. He returned to the starting rotation with San Diego, making 32 starts, his most since 1998 with the Reds.
He was traded again, almost exactly a year later (December 15, 2002) to the St. Louis Cardinals for Mike Wodnicki and Luther Hackman. Tomko won 13 games in his only season in St. Louis.
He signed with the San Francisco Giants as a free agent prior to the 2004 season and pitched with them for two seasons. Tomko's tenure with the Giants effectively ended when the team declined to offer him salary arbitration before the 2006 season.
On December 21, 2005, he agreed to a two-year contract worth a reported $8.7 million with the Los Angeles Dodgers. He pitched both as a starter and a reliever during his two seasons in Los Angeles. During his time with the Dodgers, Tomko's poor pitching and propensity to give up home runs led to Dodger fans giving him the nickname "Bombko".
He was designated for assignment on August 24, 2007, after a poor season with the Dodgers in which he went 2-11 with a 5.80 ERA in 33 games (15 starts).
After being designated for assignment by the Dodgers, he was signed by the San Diego Padres on September 4, 2007.
On January 20, 2008 he signed with the Kansas City Royals.
On June 12, 2008, the Royals designated Tomko for assignment and on June 20, 2008, he was released.
He signed with the San Diego Padres on June 27, 2008, but was released on September 1.
On February 13, 2009, the New York Yankees signed Tomko to a minor league contract with an invitation to spring training. He began the season with Triple-A Scranton. He was called up by the Yankees on May 9. Tomko was designated for assignment on July 21, 2009, to make room on the roster for Sergio Mitre. He criticized the Yankees for not using him enough, despite his excellent spring training and minor league numbers, and said his 5.25 ERA was due to a lack of use. He was released on July 29.
He then signed with Oakland Athletics. On August 17, 2009, Tomko defeated the Yankees in his first start for Oakland. He resigned with Oakland during the 2009-2010 offseason and rehabbed with the Sacramento River Cats.
On February 19, 2011, the Texas Rangers signed Tomko to a minor league contract with no invitation to spring training. On April 20, the Rangers purchased his contract from the minors and called him up. He was outrighted to Triple-A on May 27. After the 2011 season, he elected for free agency.
On February 19, 2012, Tomko signed a minor league contract with the Cincinnati Reds worth $480,000. On August 2, 2012, Tomko was released by the Reds. Tomko was 0-6 with a 3.78 ERA in 12 starts with Triple-A Louisville. In a rehab start with the Arizona League Reds, he lasted 1.1 innings, giving up 8 runs (7 earned) off of 9 hits.
Tomko signed a minor league contract with the Arizona Diamondbacks on August 14, 2012. He was assigned to the Double-A Mobile BayBears. On August 24, Tomko was called up to the Reno Aces.
In March, 2013 Tomko signed a contract with the York Revolution of the independent Atlantic League of Professional Baseball. He retired in August and joined the Kansas City Royals organization as a scout.
On March 14, 2014, Tomko agreed to a minor league contract with the Kansas City Royals. On June 3, Tomko was released by the Royals.
Tomko signed a minor league deal with the Colorado Rockies on June 10, 2014.
Tomko married Playboy Playmate Julia Schultz (February 1998) in November 2003.
Tomko is an artist, currently training with famous sports artist Opie Otterstad.
Tomko's father won a contest of over 11,000 entries in The Plain Dealer for naming the Cleveland Cavaliers NBA team. His entry stated, "The name Cleveland Cavaliers represents a group of daring, fearless men whose life's pact was never surrender, no matter what the odds."