Born in Honolulu, Hawaii, Fernandez was proud of his roots and wore uniform number 50 in honor of Hawaii being the 50th state. The theme song to Hawaii Five-O was often played before the start of each game he pitched at Shea Stadium. According to the Portuguese Heritage Foundation, Fernandez is believed to be of Portuguese descent.
Fernandez had an unorthodox pitching motion with a hesitation at the end followed by a sudden slingshot sidearm delivery. This deceptive motion, coupled with an effective curveball and a rising fastball, made him a major strikeout threat throughout his career. Fernandez' strikeouts were often commemorated by Mets fans in the outfield upper deck with taped signs marked with the letter S for Sid.
While he was popular with Mets fans, critics of Fernandez point out that his statistics were much better in pitcher-friendly Shea Stadium. In five seasons with the Mets (1986–1991 excluding 1989), his ERA was at least two runs worse on the road than at Shea.
Fernandez allowed only 6.85 hits per nine innings for his career which is the fourth-best total in history behind only Baseball Hall of Famers Nolan Ryan and Sandy Koufax and three-time Cy Young Award winner Pedro Martínez.
Fernandez' 1983 achievements in AA earned him a call-up to the Majors for September where the Dodgers were in a division race. He made his Major League debut during a blowout game on September 20. The Dodgers clinched the division so the 20-year old Fernandez started the last game of the season, losing to the San Francisco Giants.
During his time in the Dodgers organization, Fernandez fought weight problems. They traded Fernandez and an infielder to the Mets for Carlos Diaz and Bob Bailor following the 1983 season. The trade turned out to be very one-sided as both Diaz and Bailor played little for the Dodgers and were out of the Majors before . Fernandez, meanwhile, pitched for another fourteen years.
In , Fernandez again started in Tidewater but was called up in early May. This time, he showed the form that Mets observers would become accustomed to. In 170 1/3 innings, Fernandez struck out 180 batters while only allowing 108 hits. Both ratios were easily the best in the entire Major Leagues, with second place in both categories going to his teammate, Dwight Gooden (who had his best season in 1985). Fernandez' 5.71 hits allowed per nine innings are the second-best in National League history behind only Carl Lundgren's 5.65 in . Fernandez' downfall was bases on balls which, combined with poor run support, resulted in a record of only 9-9. Seven of his nine losses were in games where he gave up two or fewer earned runs Despite having the third-best record in baseball, the Mets were second-best in their division and thereby missed the postseason.
Fernandez' statistics were only average in but the Mets could do no wrong as he compiled a 16-6 record. A 12-2 start resulted in his first All-Star Game appearance and the first ever appearance by a Hawaii native In his only inning of the All-Star Game, Fernandez walked the first two batters but then struck out Brook Jacoby, Jim Rice and Don Mattingly in succession to get out of the inning. For the regular season, his success was almost completely based on home field advantage as his ERA was 2.17 at home and 5.03 on the road. Regardless of his road performance, Fernandez was one of four Mets pitchers to receive votes for the Cy Young Award — the only Cy Young Award vote of Fernandez' career. He finished a distant seventh while Houston's Mike Scott won the award in his best season.
Fernandez faded in the second half but the Mets easily won the division and he went head-to-head against Scott in Game 4 of the 1986 National League Championship Series. With a chance for the Mets to take a three-games-to-one lead, Fernandez gave up an early two-run homer and Scott coasted to a 3-1 win. The Mets recovered to win the next two games. In the World Series, Mets manager Davey Johnson feared starting a left-handed high-fastball pitcher in Fenway Park with its shallow Green Monster so Fernandez was stationed in the bullpen. Dwight Gooden faltered in Game 5, falling behind 4-0 in the pivotal game. Fernandez took over in the fifth inning and shut down the Boston Red Sox for four innings but the damage was done dropping the Mets into a three-games-to-two hole. After the legendary Bill Buckner Game 6, the Red Sox recovered to take an early 3-0 lead in the deciding Game 7. With starter Ron Darling ineffective, Fernandez entered and retired seven batters in a row including four strikeouts. With the momentum seemingly back to the Mets, they scored three runs in the sixth inning and three more in the seventh and won the game 8-5 for their second world championship.
In , Fernandez was again fantastic for the first ten weeks gaining another All-Star Game appearance — but again declined after the break, going just 3-3. A knee injury caused him to miss three weeks in August and many observers again considered his weight to be a factor. For the second year in row, his statistics were heavily skewed towards home field advantage with a 9-3 record and 2.98 ERA at home compared to 3-5, 5.05 on the road. Similar to 1985, the Mets had a better record than two division winners — but not their own division winner — so they again missed the postseason. was a reversal as Fernandez started the season miserably and then recovered later in the season. His ERA was 7.53 after three games and 5.57 in mid-May but dropped all the way to 3.32 for the All-Star break. Around that time, he went on a strikeout tear, punching out fifty batters in five games — but only managed a 2-3 record. He finished out the season well and the Mets won 100 games, making the postseason for the second time in three years. For the second time, Fernandez led the Majors in hits allowed per nine innings. For the third year in a row, home field advantage was a huge factor for Fernandez with an 8-4 record and 1.83 ERA at home compared to 4-6, 4.36 on the road. With the 1988 National League Championship Series tied, Fernandez was chosen to start the important Game 5 and he responded by pitching well for three innings. In the fourth inning, he fell apart, giving up three runs and, in the fifth, Kirk Gibson hit a three-run homer to knock him out of the game. The Dodgers coasted to a 7-4 victory and won the series in seven games. What many thought would be a Mets dynasty never materialized and they didn't make the playoffs again until a decade later.
In , Fernandez was again in the top ten in several categories but luck was not one of them as he finished the season at 9-14, the worst record of his career. He fell back into his old form of pitching well only at home, going 8-5 with a 2.41 ERA at Shea and 1-9, 4.94 on the road. was even worse as he broke his arm in spring training and returned in mid-July, only to go down again with knee problems in early September. brought a return to the statistical leaderboards for Fernandez but his team-leading fourteen wins could not save the Mets from a second straight fifth-place finish in their six-team division. was the culmination of numerous poor trades and free agency signings and resulted in the worst Mets record in recent memory. Among the many disasters, Fernandez, who had become an ace of the staff and one of the last links to the Mets recent glory days, missed half the season with another knee injury while covering first base The 1993 injury would plague him for the rest of his career. He came back to put up good numbers that season but the Mets were in dire need of drastic change and Fernandez left for free agency.
Three days after his release, Fernandez was back in the National League when the Phillies signed him. He showed flashes of brilliance — including a one-hit game over seven innings on July 26 — and went 6-1 for the Phils. He was even named N.L. Pitcher of the Month in August by going 5-0. The resurgence earned Fernandez his only opening day start in but more injuries ended his season in June and he was back on the free agent market.
On September 28, 1996, baseball took a back seat when Fernandez' father-in-law, Don Mike Gillis, was shot to death in Honolulu. Fernandez soon announced the dedication of his 1997 season to Gillis. A disturbed co-worker was eventually convicted of the murder.
Fernandez was signed by the Astros for 1997 but complained of elbow problems during spring training. After just one start, he was back on the disabled list and, after unsuccessful rehabilitation, Fernandez retired on August 1, 1997.
In fifteen Major-League seasons, Fernandez compiled a record of 114 wins and 96 losses with 1,743 strikeouts. His career ERA was 3.36 and opponents batted only .209 against him.
In , Fernandez received two votes in his only Hall of Fame ballot.
In 2000, Fernandez was the pitching coach of the semi-pro Alaska Baseball League's Hawaii Island Movers.
In February , Fernandez surprised many by showing up at New York Yankees spring training. He was given a minor league contract and pitched in one game for the Columbus Clippers on April 7. He pitched poorly and ended with a sore knee which put him back on the disabled list. He re-retired about a week later.
Fernandez has also spent a lot of time golfing in celebrity matches and making other appearances since his retirement. He and his wife, Noelani, operate the Sid Fernandez Foundation which awards four $5,000 college scholarships each year to seniors from the Fernandezes' alma mater, Kaiser High School
On December 20, 2007, Fernandez's name appeared in the unsealed Kirk Radomski affidavit. The affidavit details Radomski receiving a $3,500 check from Fernandez dated February 2005, but the affidavit does not specify what the check was for. Fernandez was one of only four baseball players listed in the affidavit that was not referenced in the Mitchell Report, the others were Rick Holyfield, Pete Rose Jr. and Ryan Schurman.