Matz was born and raised on Long Island and played baseball for Ward Melville High School in East Setauket, New York.

The start of Matz's professional career was delayed two years due to Tommy John surgery needed for a tear in the ulnar collateral ligament of his throwing elbow.

Matz first drew attention from scouts in the New York Mets organization during a baseball showcase tournament in 2008,[7][6][17] and their interest in him continued to grow in subsequent years.

[3][6][7] Some Mets scouts attributed his drop to his playing high school baseball in the Northeast, an area which does not traditionally produce much high-round talent.

[6] Matz, who was seeking a $1.1 million signing bonus,[24][25][26] received inquiries from three other teams during the draft,[24] including the Los Angeles Dodgers and Chicago White Sox, but he ultimately passed on them with the hopes of making the Mets.

[7] Steven Matz was called up to the New York Mets to join a six-man pitching rotation for the 2015 season, along with Bartolo Colón, Jacob deGrom, Matt Harvey, Jon Niese, and Noah Syndergaard.

[50][52] Additionally, using a bat gifted to him by Las Vegas teammate Matt Reynolds,[47][48][52] Matz recorded three hits as a batter and drove in four runs.

[31][48] In his second start, Matz pitched six scoreless innings while striking out eight and allowing two hits in a win against the Los Angeles Dodgers.

[55] Shortly afterward, Matz was placed on the disabled list for two months due to a partial tear in the latissimus muscle in his left side.

[82] Matz opened the season with the worst start of his career to date,[83] surrendering seven runs before getting pulled in the seventh inning of a 10–3 loss to the Miami Marlins.

[88] He was named National League Rookie of the Month for May,[88][89] becoming only the third Mets player to receive that distinction, along with Justin Turner in 2011 and Jacob deGrom in 2014.

[35][106][107] Matz rehabbed his left shoulder in September,[108][109] and tried to return to the pitching rotation several times,[87] with the team hoping to bring him back before the end of the season in the Mets bullpen.

[121] At the start of the season, the MLB marketed the Mets pitching rotation of Matz, Jacob deGrom, Matt Harvey, and Noah Syndergaard as the "Four Horsemen of Queens", with a commercial suggesting they could be "the best staff in baseball".

[106][118][123] Team doctors believed at the time it might have been a side effect of his bone spur surgery the previous fall, and prescribed platelet-rich plasma injections and rest.

[106][112][138] After it was announced Matz would require surgery, it was revealed he had been skipping bullpen sessions between starts,[106][112][139] and limiting the use of his slider to lessen the strain on his elbow.

[153] The game ended with a 25–4 loss, the most lopsided in Mets' history,[119][153] and caused Matz's season ERA to rise from 3.79 to 4.35,[30][153] compared to 3.21 before the All-Star break.

[56] On September 1 against the San Francisco Giants, Matz struck out a career-high 11 batters over seven innings, allowing three hits, one run, and one walk.

[166] Matz prepared for 2019 by practicing light throwing nearly the entire off-season, starting in mid-October and taking only two weeks off, a method inspired by that of fellow Mets pitcher Jacob deGrom.

[180] After missing two weeks in May due to radial nerve discomfort in his left forearm,[181][182] Matz had a career-best 9.07 K/9 by May 23, and his walk rate of 2.62 per nine innings was 0.77 better than his average the previous year.

[199] This included his first complete-game shutout on July 27, throwing 99 pitches,[195][200] recording seven strikeouts, and allowing five hits as the Mets defeated the Pittsburgh Pirates 3–0.

On January 27, 2021, the Mets traded Matz to the Toronto Blue Jays in exchange for right-handed pitchers Sean Reid-Foley, Yennsy Díaz, and Josh Winckowski.

[225] Matz had a strong fastball since an early age, but during his minor league career he made a conscious effort to develop his curveball and work it into his pitching repertoire in place of his slider.

In a 2014 scouting report for the website, Jeff Moore wrote, "if the change-up continues to develop, he'll have three offerings he can throw any hitter at any time",[225] along with the fastball and curveball.

Moore added Matz's fastball velocity comes with minimal effort and that he positions it well on the outer half of the strike zone for right-handed hitters.

"During the 2019 season, Matz made a minor adjustment to his pitching position, moving from the first base side of the mound to the middle, giving him greater comfort and improving his performance as a result.

[138] Matz proposed to Cain on November 12, 2016, at a friend's house in Yaphank, New York, on a dock overlooking a pond that he called one of his "favorite places on Long Island".

[233] He was also the Mets' 2018 nominee for the Roberto Clemente Award, which honors baseball players for community involvement, philanthropy, and positive contributions on and off the field.

[231] In May 2018, Matz participated in The Viscardi Center's Celebrity Sports Night to raise funds for the Albertson-based organization, which provides schools and training for children and adults with severe physical disabilities.

[32][238] He was raised as a non-practicing Lutheran, but after attending a Bible study group during his time in the minor leagues, he joined a non-denominational church, Calvary Chapel, in Queens.

[33] Matz has donated equipment to the Three Village Little League, where he played as a child,[32] and has participated in baseball clinics at his Ward Melville High School alma mater.