The Dallas Cowboys entered the 2005 NFL Draft with two first round draft choices. Spears was considered by head coach Bill Parcells to be the key to the team’s eventual move to a 3-4 defense and wanted to take him with the first selection (11th overall pick). Owner/GM Jerry Jones overruled Parcells and drafted DeMarcus Ware instead, who the team thought would not be available later in the draft. Spears was eventually selected by the Cowboys with the 20th overall pick.
Spears suffered from injury problems in his first two training camps. In his first training camp in 2005 as a rookie, He sprained his right knee and ankle while straining his calf. What could have been a serious injury was only a minor setback for him. He missed the first three preseason games but bounced back in time for the final preseason game and recorded three tackles and a sack.
He started the season as a backup in the team’s 4-3 defense and eased into a right defensive end starting role midway through the season, to earn All-Rookie honors from Pro Football Weekly. His 35 tackles were good for 12th on the team – third among linemen and rookies while his six quarterback pressures were fourth. He added 1.5 sacks and a pass defensed.
In his NFL debut at the San Diego Chargers, Spears was in the defensive line rotation but did not record a tackle. He recorded his first sack – one he split with Greg Ellis – and his first career tackle against the Washington Redskins. Spears then led all defensive linemen with four tackles at the San Francisco 49ers. He helped limit the Philadelphia Eagles to 129 yards, including just 19 rushing, the fourth lowest in franchise history. Spears earned his first NFL start at defensive end at the Seattle Seahawks and responded with three tackles and a batted pass while helping limit Shaun Alexander, the NFL’s leading rusher to 21 carries for 61 yards (2.9 avg.). He officially moved into the starting lineup full-time against the Arizona Cardinals and led all linemen with four tackles. Spears logged his third career start in Philadelphia and posted a season-high seven tackles to lead all linemen and finish second on the team. He had a three-tackle game at the New York Giants. Against the Kansas City Chiefs, Spears recorded four and a fumble recovery. With 1:16 remaining in the first half and Dallas trailing 14-10, Spears scooped up a Trent Green fumble at the Cowboys 15-yard line and returned it 59 yards to the Chiefs 26-yard line. Three plays later, Drew Bledsoe completed a pass to Jason Witten for a touchdown and a 17-14 halftime lead. It was the eighth-longest return in franchise history and the longest ever by a Dallas rookie. In Washington, Spears recorded three tackles. In the season finale against the St. Louis Rams, he recorded two tackles and his first solo sack of the season.
Before the 2006 season, Spears underwent minor knee surgery to repair meniscus damage, which forced him to miss the first two weeks of training camp. The Cowboys moved to a 3-4 defense and Spears earned the starting role at left defensive end, finishing ninth with 48 tackles (31 solo), one sack, three pass deflections, two tackles for loss and tied for fifth on the squad with three quarterback pressures.
In the season opener at the Jacksonville Jaguars, Spears had two tackles and a tackle for loss. In Week two’s Sunday night game against the Redskins, Spears recorded his only sack of the season, sacking quarterback Mark Brunell in the first quarter, he also recorded two tackles. After the Cowboys’ bye week, Spears led all defensive linemen with four tackles against the Tennessee Titans. The Titans, in rookie quarterback Vince Young first career start, only rushed 27 times for 78 yards. Spears recorded two tackles the next week in Philadelphia, where he also broke up a pass at the line of scrimmage. Against the Houston Texans, Spears recorded three tackles and a quarterback pressure. He recorded three tackles against the Giants in a Monday night loss at Texas Stadium. At the Carolina Panthers, in a Sunday night victory, Spears had two more tackles and a quarterback pressure. He had five tackles the following week in Washington, followed by four tackles in Arizona. Against the eventual Super Bowl champion Indianapolis Colts, Spears had three tackles and one for loss, limiting the Colts to a season-low 14 points. In the annual Thanksgiving Day game against the Tampa Bay Buccaneers, he had three tackles. He had two tackles in the Cowboys’ 23-20 win over the Giants at the Meadowlands. The following week Spears recorded a season-high six tackles against the Saints in a home loss. He recorded four tackles in a road win against the Atlanta Falcons, helping provide pressure on the defensive line that sacked Falcons quarterback Michael Vick four times. Spears had three tackles in the Cowboys’ Christmas Day loss to the Eagles and did not record a stat in the season finale against the Detroit Lions. In his postseason debut, Spears had one tackle and led the team with two quarterback pressures against the Seahawks.
In 2007, he appeared in all 16 regular season game and Dallas’ lone post season game. For the season Spears recorded 32 tackles (19 solo), two forced fumbles, and a career-high two sacks and three passed defensed on the season.
In the season opener against the New York Giants, Spears recorded one tackle (one solo). During week two at the Miami Dolphins, he recorded two tackles (two solo). In the week three victory over the Chicago Bears, Spears recorded two tackles (one solo). During the week four victory over the St. Louis Rams, he recorded two tackles (one solo), and 0.5 sack. In week five at the Buffalo Bills, Spears recorded two tackles (one solo). During the Cowboys first loss of the season, to the New England Patriots, he recorded four tackles. In the week seven win over the Minnesota Vikings, Spears did not record any stats. During week nine at the Philadelphia Eagles, he recorded one tackle and 0.5 sacks and a forced fumble. In the week 10 win over the New York Giants, Spears recorded two tackles (two solo). During the week 11 win over the Washington Redskins, he recorded one tackle. In the week 12 victory over the New York Jets, Spears recorded two tackles and one pass defensed. During the week 13 victory over the Green Bay Packers, he recorded one tackle, and a pass defensed. In the week 14 win over the Detroit Lions, Spears recorded one tackle. During a week 15 loss to the Eagles, he recorded three tackles (two solo). In a week 16 victory over the Carolina Panthers, Spears recorded one tackle and one pass defensed. During the season finale against the Washington Redskins, he recorded a season high five tackles (four solo), one sack, and one forced fumble.
In the Cowboys lone playoff game against eventual Super Bowl XLII champion New York Giants, Spears recorded two tackles.
Marcus Spears started 15 games of the 16 games he played in the 2008 season. He registered 35 tackles (with 23 being solo, and 12 being assisted tackles), a sack, and a forced fumble.
In 2009, he started 14 games and recorded 50 tackles (2 for losses), a career-high 2.5 sacks and a career-high tying 16 quarterback pressures and 2 pass breakups.
Spears played a key role in the playoffs, registering a tackle and three pressures in the wildcard playoff win against the Philadelphia Eagles, as the defense did not allow a rushing first down for the first time in postseason team history. He had a postseason career-high six tackles and a pressure in the divisional game against the Minnesota Vikings.
After playing in only eight games (7 starts), his streak of 88 consecutive games played ended on November 10, when he was placed on the injured reserve list, with a left calf injury. Playing mostly in running downs, he registered 24 tackles (1 for loss) and 3 quarterback pressures.
Spears signed a 5-year, $19.2 million contract during the off-season. He injured his groin in the second preseason game and missed the rest of the preseason.
New defensive coordinator Rob Ryan selected Kenyon Coleman and Jason Hatcher as the starters at defensive end, relegating Spears into a reserve role for the first time in his career. He was used mostly on running downs and registered 29 tackles (one for loss), 15th on the team and fifth among defensive linemen, one sack, two pass deflections and one quarterback pressure.
In 2012, he again had a reserve role, but got a chance to start in six games because of injuries. Spears finished the season with 35 tackles (2 for loss) and 1 sack. He missed one game with a knee injury.
On March 13, 2013, Spears was released because of a degenerative knee condition and he was not seen as a good fit in the team’s plan to change to a 4-3 defense. While he never developed into a Pro Bowl player like fellow 2005 first-round pick DeMarcus Ware, he was durable, an excellent locker room presence and fulfilled his primary responsibility in the 3-4 defense, which was to create rushing lanes for the outside linebackers.
On March 15, 2013 Spears signed a two-year, $3.55 million deal with the Baltimore Ravens. On October 30, 2013, he was waived after registering 10 tackles as a reserve defensive end.