In 2013 he impressed at Kingsport with an 8-1 record. In 2016 he was at St. Lucie where he went 10-9 with a 3.25 ERA in 25 starts. In 2017 he was promoted up to a AA Binghamton going 6-1 with a 1.66 ERA. That was enough for the Mets to have him skip AAA & go right to the big league club, who's staff was struggling.
Mets Career: Flexen made his MLB debut on July 27th 2017, with a start in San Diego. The Padres got him for four runs on five hits, in three innings of work, taking his first career loss.
Trivia: He became the fifth Met pitcher in team history to serve up a HR to his first career hitter.
After a no decision, he earned wins against the Texas Rangers & Philadelphia Phillies. He gave up at least two runs in each of those games getting to the 5th & 6th innings respectively. On August 23rd, he had his best outing of the year, pitching six innings, allowing two runs with five strike outs, in a win over the Arizona Diamondbacks at home.
That was followed by his worst outing, it came against the eventual World Champion Astros at Houston. He was pounded for seven runs on nine hits in just four innings of work. This led to him getting placed in the bullpen for most of September.
On September 25th he got a start against the Atlanta Braves, his first five innings were good, but then in the 6th it all fell apart. Overall he allowed four runs in five innings of work resulting in another loss. On October 1st he ended the season with another debacle, giving up six runs (five earned) in a 11-0 loss at Philadelphia.
On the year he was 3-6 with a 7.8 ERA, 36 strike outs & 35 walks in 48 innings of work.
In 2018 he made it back to the Mets staff by late May. On May 26th in two innings he allowed seven runs, although just three were earned. He also gave up eight hits in a 17-6 loss to the Brewers in Milwaukee.
He was sent right back down to AAA.
He returned in in late June, appearing in two games. On July 8th he had yet another bad outing, allowing five runs on five hits in three innings pitched in a 9-0 loss to the Tampa Rays.
In August he underwent knee surgery & ended his season. He ended at 0-2 with a 12.79 ERA in four games.
In 2019 Flexen made a start on April 20th, against the Cards in St. Louis. He was rocked for five earned runs & seven hits in 4.1 innings. He was back at AAA making a May 4th appearance before going back to Syracuse. He would return in June & take a loss to the Cardinals again, in his first game back. He would make six more appearances, finishing off a win & a loss.
He went back to AAA Syracuse, where he ended the season at 5-3 for the Chiefs. That winter he was designated for assignment.
Korean Baseball: In December of 2019, he was signed in the Korean Baseball Organization's Doosan Bears, that years Baseball Champions. In Korea, Flexen found success as a pitcher going 2-0 with a 2.92 in his first six starts. In 2020, he averaged a 10.2 strikeouts per nine innings, while posting a 3.01 ERA.
Return to MLB: In 2021 he signed with the Seattle Mariners, returning to MLB. On April 3rd, he beat the San Francisco Giants, shutting them out for five innings, earning his first win in the majors in almost two years.
He was rocked in Minnesota in his next start, allowing six runs in five innings. Flexen won his next three starts, then suffered another beating, as the Padres roughed him up for 8 runs in 1.2 innings. By mid June he was 5-3 with a 4.68 ERA, making 11 starts.
In four seasons, Flexen is 8-14 with a 6.49 ERA, he struck out 85 batters walked 66 in 127.2 innings pitched in 38 appearances, 22 starts. (thru June 14th 2021)