Bobby Parnell was born on September 9, 1984 in Salisbury, North Carolina. His father, also named Robert, and his grandfather were both firefighters. At East Rowan High School in Granite Quarry, North Carolina, Parnell was primarily a third baseman. Parnell converted from shortstop to pitcher as a freshman with the Charleston Southern University Buccaneers. Despite still struggling to control his pitches even after three years of pitching in college, the New York Mets took the advice of Parnell's former American Legion coach and selected Parnell in the 9th round of the 2005 Major League Baseball draft with the 269th overall pick.
Parnell worked as a starter for the minor league teams in the Mets organization, pitching for the Brooklyn Cyclones, St. Lucie Mets, Binghamton Mets, and New Orleans Zephyrs. In three different leagues, Parnell has been named a mid-season All-Star. He earned the honor in the New York–Penn League in 2005, the Florida State League in 2007, and the Eastern League in 2008.
Parnell earned Eastern League All-Star honors again in 2008. Despite having a poor record since being promoted to the Triple-A New Orleans Zephyrs, Parnell was rewarded with a call-up to the New York Mets on September 1, 2008. He made his Major League debut on September 15.
Parnell earned a spot in the Mets bullpen to start the 2009 season. He got his first win May 8, 2009 against the Pirates; he pitched a scoreless eighth inning. On August 5, he got his first major league hit in his first career at-bat, as he pitched three innings for his first career save. After Mets pitcher Jon Niese suffered a season-ending injury in the same game, Parnell was tabbed to replace Niese in the rotation. He made his first Major League start on August 8 against the San Diego Padres. He earned his first Major League win as a starter in his second start on August 14, pitching six innings, giving up only three hits, and striking out seven. On September 4, Parnell shut down the Cubs that battered him in his last start. Parnell pitched a career-high seven innings, shutting out the Cubs on five hits while striking out seven.
On June 29, 2010 Jenrry Mejía was sent back down to Double-A Binghamton and the Mets called up Parnell to take his spot in the Mets bullpen.
Parnell, in his short major league career, has already developed a reputation for being a flamethrower. His fastballs consistently touch the high-90s, sometimes eclipsing 100 MPH. On August 18, 2010, in a game against the Houston Astros, Parnell hit 102.5 MPH on the radar gun, the fastest pitch in the Major Leagues in 2010 up to that point. Cincinnati Reds rookie Aroldis Chapman set the new mark on August 31, 2010 at 103.9 MPH.
On September 21, Parnell was shut down for the remainder of the 2010 season with an inflamed pilca in his right elbow.
On April 21, 2011, Parnell was placed on the DL due to a clot in his finger.
On May 30, 2011 he returned to the club as José Reyes was placed on the Bereavement list due to the death of his grandmother.
On June 29, 2011 he made a relief appearance against the Tigers. In an at bat against Miguel Cabrera, Parnell threw seven straight pitches over 100 mph.
To begin the year Parnell was used as a middle reliever, yet as injuries mounted he was tabbed to be the closer. On June 28, 2012, Parnell recorded his first save of the year against the Los Angeles Dodgers.
Parnell was named the Mets closer for the 2013 season due to an injury to Frank Francisco. Parnell recorded 22 saves before being shut down for the rest of the season on July 30.
Parnell was named the Mets closer for the 2014 season. On opening day, he blew his first save opportunity. The next day, it was reported that Parnell tore his ulnar collateral ligament and he underwent Tommy John surgery shortly thereafter, ending his season.
On January 7, 2015, Parnell and the Mets agreed to a one-year deal worth $3.7 million, and avoided arbitration. Parnell made his return to the Major Leagues in a June 13 win against the Atlanta Braves at Citi Field. Parnell pitched well in the relief appearance and said of his return, "It sent chills up my spine." However, Parnell's fastball velocity was noticeably lower than before his surgery and manager Terry Collins described Parnell as looking "rusty."
On August 18, 2015, after pitching poorly in a weekend series against the Pittsburgh Pirates and taking two losses, (along with a 14.14 ERA in his last 10 appearances) he was placed on the 15-day disabled list with arm fatigue.
On February 18, 2016, Parnell signed a minor league contract with the Detroit Tigers, and was invited to spring training. On March 28, Parnell was released by the Tigers. On March 30, the Tigers re-signed Parnell to a minor league contract. On June 1, Parnell was recalled by the Tigers. He was designated for assignment on August 18. On August 21, 2016, the Tigers released Parnell.
In December 2016, Parnell signed a minor league contract with the Kansas City Royals. He exercised an opt out clause in his contract on June 2, 2017, and became a free agent.
On June 5th, 2017, Parnell and the White Sox agreed to a minor league deal. He was released on July 20, 2017.
Parnell throws primarily a hard four-seam fastball in the mid-to-high 90s with strong tailing action. His secondary pitch and main breaking ball in 2011 was a high-80s slider, but in 2012 he dropped the pitch in favor of a mid-80s knuckle curve. He has also toyed in the past with a splitter, but he rarely uses it. For 2012, he also developed a two-seam fastball.
On January 29, 2010, Parnell proposed to his longtime girlfriend Maegan Walker in Charleston, South Carolina. The couple was married on November 11, 2011 at a Methodist church in Concord, North Carolina. In 2010, Parnell and his then-fiancée adopted a golden retriever–yellow lab mix from North Shore Animal League which they named "Abby." As of 2010, the couple was living in Parnell's hometown of Salisbury. The couple in January of 2016 welcomed twin daughters Ava and Caroline.