Laidlaw attended Hawthorne High School. He accepted a football scholarship from for Stanford University. As a sophomore backup, he averaged 5.6 yards per carry, rushing 48 times for 268 yards. As a junior, he was named the starter at fullback, leading the team in rushing with 639 yards and 4 touchdowns, while making 19 catches for 155 yards and one receiving touchdown. The next year he led the team again in rushing with 636 yards and 4 touchdowns, also posting 14 catches for 135 yards and one receiving touchdown. He finished fourth in the school career rushing list, with 1,543 rushing yards, while playing on pass-oriented offenses. He also had 40 receptions for 351 yards.
Laidlaw was selected by the Dallas Cowboys in the fourteenth round of the 1975 NFL Draft, also known as the Dirty Dozen draft. As a rookie, he was used mainly in short yardage situations, until suffering a mid season knee injury that placed him on the injured reserve list. His running style reminded the media and observers of Walt Garrison. In 1976, he replaced an injured Robert Newhouse in the season opener against the Philadelphia Eagles, rushing for 104 yards on 19 carries, along with 7 receptions for 66 yards. He missed 3 games with severe rib bruises he suffered against the Baltimore Colts and his return was also hindered by a bout with the flu. Against the Atlanta Falcons he rushed for 88 yards on 15 carries. He finished with 424 rushing yards, 325 receiving yards and 84 scored points. In 1977, he suffered a hamstring pull in training camp which limited his preseason action and kept him from seriously competing for the starting fullback job with Newhouse. As a backup that was used mainly on passing situations, he made just 5 receptions for 15 yards and 9 carries for 60 yards. He was a part of the Super Bowl XII winning team. In 1978, an injury to Newhouse late in the season allowed him to become an important contributor in the final push toward a second straight Super Bowl appearance. Against the Washington Redskins on Thanksgiving he had 16 carries for 122 yards and 2 touchdowns, including 103 first-half rushing yards and a 59-yard run to set up a field goal. Two weeks later against the Philadelphia Eagles, he scored on a 44-yard screen pass reception and on a one-yard run. In the first-round playoff game against the Atlanta Falcons, he had 17 carries for 66 yards and 2 touchdowns. The following week in the conference championship game against the Los Angeles Rams, he had a four-yard scoring reception that put the Cowboys ahead 14-0 in the fourth quarter. Laidlaw ended up scoring 7 touchdowns in the 6 games prior to Super Bowl XIII. In 1979, he was set to contend with Newhouse for the starting fullback job, but Laidlaw suffered a pulled hamstring in training camp and was unable to make a serious challenge. After Newhouse was limited with a leg injury he sustained in the season opener, him and Laidlaw were used as interchangeable play messengers during the first half of the season. Laidlaw could never regain his full strength however, because of a cyst he had behind his left knee that was removed until the offseason. On August 26, 1980, he was released after the team gave his roster spot to rookie Timmy Newsome. During his time with the Cowboys, although his effectiveness was limited by a series of injuries, he still managed to be a key backup and a part of three Super Bowl teams.
