Christensen was born in Bellefonte, Pennsylvania, on August 3, 1956, to Ned Jay and June Christensen. His father was working on a doctorate degree at Pennsylvania State University at that time. After teaching in West Virginia, his father was offered a professorship in Eugene, Oregon, when Todd was 5 and the family relocated.

Athletically, Christensen’s early desire was to continue excelling in track and field as he had when he set the world records as a 9-year-old boy. “Puberty and adolescence helped me realize that I was not as fast as I had thought,” he recalled. “My body went a different direction and that was when I started leaning towards football.” He graduated from Sheldon High School in Eugene, and then attended Brigham Young University (BYU) in Utah.  At BYU, Christensen was a four-year starter (1974–1977) at running back, led the team for three consecutive seasons in receiving and was an All-Western Athletic Conference selection in 1977. His career numbers while at BYU: 152 receptions, 1,568 yards and 15 touchdowns. He graduated with a degree in social work in 1978 before embarking on his pro career.

Christensen began his NFL career as a second-round draft pick of the Dallas Cowboys in the 1978 NFL Draft. While playing fullback and leading the team in rushing, he broke his foot in the final exhibition game, so he was placed on injured reserve and couldn’t play a down in a season the team went on to play in Super Bowl XIII. The next year the Cowboys wanted to convert him to tight end, but he didn’t agree with the move after working one week in his new position, so he was waived at the end of training camp.  Christensen was claimed off waivers by the New York Giants but only played in one game, and lasted two weeks with the team.

After being unclaimed, he was signed by the Oakland Raiders in 1979 and became a key player on special teams. After three seasons of unspectacular statistics (including the Raiders’ Super Bowl winning campaign in 1980), Christensen broke out in 1982, catching 42 passes for 510 yards and four touchdowns during the strike-shortened season, helping the Raiders to the best record in the NFL. The next year, Christensen caught 92 passes for a career high 1,247 yards and 12 touchdowns and earned the first of his five trips to the Pro Bowl for his efforts. His total catches led the NFL, making him the second tight end to ever do this (Kellen Winslow was the other). The Raiders finished the season with a resounding 38–9 victory over the Washington Redskins in Super Bowl XVIII. Christensen topped 1,000 yards again in 1984, catching 82 passes in the process. He hit 80 receptions again the following year, missing 1,000 yards by just 13 yards. The 1986 NFL season was Christensen’s last big one statistically. He ended the year with a career-high, league-leading 95 receptions for 1,153 yards and eight touchdowns. He also became the first tight end in history to catch 90 passes in each of two consecutive seasons.

Christensen’s 1987 campaign was cut short, but in 12 games he still managed to catch 47 balls (a little fewer than four per game). His 663 yards averaged to 14.1 yards per reception, a career high in seasons where he caught at least 40 passes. In Christensen’s final year, he missed more than half the season with injuries. He only caught 15 passes, with none going for touchdowns, and then he retired from pro football.  In his career, Christensen caught 461 passes for 5,872 yards and 41 touchdowns. In eight postseason games, he caught 31 balls for 358 yards and only one touchdown. He led the league in receptions twice, and his 349 receptions from 1983 through 86 (four seasons) was an NFL record.

Following his football career Christensen had tryouts with the Oakland Athletics and the Anaheim Angels but found his niche in Masters Track and Field where he set an age-group World Record in the Heptathlon and was the top decathlete in the world for ages 45-and-over. Christensen became a broadcaster, co-hosting the second half of the first season of American Gladiators with Mike Adamle. He later joined the NFL on NBC as a color commentator from 1990 to 1994, teaming up with Charlie Jones for the first four years (see List of NFL on NBC commentator pairings).

In 1994, Christensen guest-starred on an episode of Married… with Children titled “Kelly Knows Something.”  Christensen did color commentary for ESPN’s college football coverage before moving to MountainWest Sports Network. Christensen would remain with “the mtn.” until the network shut down in June 2012. Christensen was announced as the new analyst for CBS Sports Network Navy games in August 2012. In 2000, he was inducted into the Oregon Sports Hall of Fame.  Christensen died at age 57 on November 13, 2013, from complications during liver transplant surgery at Intermountain Medical Center in Murray, Utah, near his home in Alpine. He had battled liver disease and related illnesses for about two years, though his son, Tory, said his liver issues began with a “botched” gall-bladder operation 25 years earlier.