Youngblood was drafted by the Los Angeles Rams in the first round of the 1971 NFL Draft. He was the 20th overall pick in that draft and signed a 3-year $105,000 contract including a $30,000 signing bonus. That season he backed up Deacon Jones at left defensive end and started four games when Jones was sidelined with a severely sprained arch. He was named All-Rookie by Football Digest and after the season Jones was traded to the San Diego Chargers. In 1972 the left defensive end position was Youngblood's as he led the Rams defensive linemen in tackles with 70, and started 11 of the 14 games he played, recording six sacks.
In 1973 Youngblood was a Second-team All-pro selection and went to the first of his seven Pro Bowls and led the Rams with 16½ sacks. The Ram defense led the NFL in fewest yards allowed and fewest rushing yards. He was voted the Rams defensive lineman of the year by the Rams Alumni Association. Beginning in the 1973 season, the Rams added the unrelated Jim Youngblood to its roster, so from that time on, both Youngbloods had the unique distinction of having their entire name on the back of their jerseys, the given name appearing above the family name. The following year, 1974, the Rams again led the NFL in rushing defense and Youngblood led the Rams with 15 sacks while being voted a consensus First-team All-Pro. The Rams advanced to the NFC Championship game, losing 14-10 to the Minnesota Vikings.
Youngblood was honored as the NFC Defensive Player of the Year by United Press International in 1975 and Pro Football Weekly named Youngblood the NFL defensive lineman of the year. For the third consecutive season Youngblood led the Rams in sacks (15) and was a consensus All-pro again, repeating his 1974 honor. In a December, 1975, 35-23 playoff win over the St. Louis Cardinals, Youngblood pass-rushed Cardinals offensive lineman Dan Dierdorf, penetrated into the backfield, then tipped and intercepted a pass by Jim Hart, returning the interception 47 yards for a touchdown. Later in the game, Youngblood forced a fumble that was recovered by teammate Fred Dryer, blocked an extra point attempt, and sacked Hart to stop a Cardinals drive.
"I'd love to watch Jack Youngblood play. His tan arms hanging out of his sleeveless jersey, he'd put those pipes on the ground, and even at 240 pounds, he would show great moves and natural strength for an undersized player. He was extremely quick, had underrated strength, and he got great leverage against the tackles.″
ESPN analyst Sean Salisbury
Youngblood repeated his NFC Defensive Player of the Year Award in 1976 while co-leading the Rams in sacks with 14½ and being a consensus First-team All-pro for the third straight season. The following year, 1977, Youngblood was voted to his fifth consecutive Pro Bowl and a consensus All-NFC selection and Second-team All-pro while leading the Rams in sacks for the fifth straight season. In 1978 the Rams led the NFL in total defense and Youngblood was a consensus First-team All-Pro for the fourth time in five years.
One of the athletic feats for which Youngblood is best known, is that of playing the entire 1979 playoffs, including Super Bowl XIV, with a fractured left fibula. He also played in the 1980 Pro Bowl with the injured leg, a week after the Super Bowl. In the playoffs, Youngblood sacked Dallas Cowboys quarterback Roger Staubach near the sideline in the waning moments of the divisional playoff game versus the Cowboys. Playing with the fractured leg was noted by Sports Illustrated in their Top 10 list of athletes playing in pain. For that and other achievements Jack was dubbed the “John Wayne of football” by Jim Hanifan and echoed by Hall of Fame coach John Madden.The NFL Network series NFL Top 10 selected Youngblood's performance in the 1979 playoffs as top on its list of the “Gutsiest Performances″ of all-time.
For the 1979 season Youngblood had a career-high 18 sacks and was a consensus First-team All-pro for the fifth time. He was voted to his seventh consecutive Pro Bowl. In 1980 he was Second-team All-pro and First-team All-NFC while leading the Rams with 11½ sacks. In 1981 Jack led the Rams with 12½ sacks and was the Rams outstanding defensive lineman. In the off-season, prior to the 1981 season, Jack had emergency surgery to remove a hot-dog sized blood clot from under his left arm. It was a result of repeated trauma to a nerve in his arm that blocked the flow of blood. Despite the broken leg and numerous other injuries, Youngblood played in 201 consecutive games, a Rams team record; and only missed 1 game in his 14-year NFL career. He played in seven straight Pro Bowls, 5 NFC Championships, and one Super Bowl. He was also the Rams defensive captain from 1977 through 1984 and was voted the Dan Reeves award 3 times, which is awarded to the team's MVP. He had 151½ career sacks and led the Rams in sacks nine times despite playing first in assistant Coach Ray Malavasi's stop-the-run-first defensive scheme and then in his final two seasons in Defensive Coordinator Fritz Shurmer's 3-4 two-gap scheme which limited some pass rush opportunities to make sure the opponent's running game was handled.
Youngblood faced a challenge in 1983 when the Rams adopted Shurmer's 3-4 defense. Critics thought Youngblood might be too small to play that position, yet he performed in it well (recording 10½ sacks in 1983 and 9½ sacks in 1984 while Rams were among the NFL's best defenses at stopping the run) despite being considered undersized. Among the standout games in Youngblood's final two seasons were the opening game of the 1983 season, against the New York Giants in which Youngblood recorded two sacks; and the 1983 season finale against the New Orleans Saints. In the Saints game Youngblood recorded 10 tackles, two sacks, recorded a safety and was named the NFL Defensive Player of the Year by Pro Football Weekly for the effort. In Week 5 of 1984 against the New York Giants, Youngblood recorded two sacks, drew three holding calls and was named NFC Defensive Player of the Week by the NFL. Then, in Week 10, against the St. Louis Cardinals, Jack dominated the game sacking Neil Lomax three times and drawing three holding calls, and blocking a potential game-tying field goal on the game's final play to preserve a 16-13 Rams win.
His streak of consecutive games played ended in Week 15 of the 1984 season, when Youngblood had to sit out his first football game since being a collegiate player in 1970. He had suffered a ruptured disc in his lower back two weeks earlier. Despite the injury, he returned for the season finale against the 49ers and the playoffs. He attributed his ability to play to a series of back adjustments that allowed him more freedom of movement, even though team doctors told Youngblood he was out for the season and needed surgery. He was voted the Rams' recipient of the 1984 Ed Block Courage Award by “representing everything that is positive about professional football and serving as an inspiration in their locker rooms being a positive role model in his communities”.
When Youngblood retired on August 27, 1985, he asked his career to be remembered for “dignity, integrity, respect and pride″.