Runnels debuted in World Championship Wrestling (WCW) in 1990 as Alexandra York, a laptop-carrying accountant who led an alliance known as the York Foundation, a group of wrestlers with finance-themed gimmicks. She worked at WCW for two years.
Runnels debuted in the World Wrestling Federation at the Royal Rumble in 1996 as Marlena. She debuted as a coolly unconcerned and indifferent manager. With her cigar-smoking, seductive, nonchalant and mysterious gimmick, she acted as the manager of her then-husband Goldust. During a feud between Goldust and Hunter Hearst Helmsley over the Intercontinental Championship on February 16, 1997 at In Your House 13: Final Four, Marlena was choked by Chyna, who debuted as a plant from a ringside seat and later became the bodyguard for Helmsley. Later, Brian Pillman feuded with Goldust over Marlena. As part of the angle, Pillman won Marlena in a match and sent Goldust a picture of her handcuffed in a bed. Pillman, however, died in the middle of the storyline. Originally, the plan was to have the rivalry culminate in an on-air wedding where Marlena would leave Goldust for Pillman. After the Marlena-Goldust alliance dissolved, Goldust revealed Luna Vachon as his new manager.
Marlena later re-emerged in 1998, under her real name, Terri Runnels, as the on-screen girlfriend of Val Venis. When Runnels claimed to be pregnant with Venis' baby, he dumped her. She later joined forces with Jacqueline, who had just ended her alliance with Marc Mero, to form the Pretty Mean Sisters (P.M.S.). They later formed an alliance with D'Lo Brown and Mark Henry, accompanying them to the ring for a match against Venis and The Godfather in December at Rock Bottom: In Your House. In January, Runnels claimed to have suffered a miscarriage after she was knocked off the ring apron by Brown. Weeks later, Brown discovered that Runnels had never been pregnant. P.M.S. gained a "love slave" named Meat in May. As a part of the storyline, P.M.S. used Meat for his body, forcing him to have sex with them. The alliance, however, broke up by July, when Jacqueline became frustrated with Runnels' infatuation with Meat.
In late 1999, Runnels arranged the Terri Invitational Tournament, a best-of-five series, between Edge and Christian and The Hardy Boyz, with the winner receiving her managerial services and the sum of $100,000. The Hardy Boyz won the tournament, which culminated in a ladder match at No Mercy (with the prize money suspended above the ring in a bag). Runnels spent the next few months as their manager until she was injured when Bubba Ray Dudley powerbombed her through a table. She returned on February 27, 2000 at No Way Out, and cost the Hardy Boyz a match. She then became the manager of Edge and Christian, and the storyline originally called for her to come between the duo, resulting in them becoming singles wrestlers with Runnels managing Christian. The storyline, however, was abandoned, and on an episode of SmackDown!, Edge and Christian—who would now continue wrestling as a tag team—revealed they no longer wanted Runnels's services. She, therefore, slapped Edge, who responded by spearing her.
Runnels began a rivalry with The Kat, and at WrestleMania 2000, Runnels (accompanied by The Fabulous Moolah) defeated The Kat (with Mae Young) in a catfight. Val Venis was the special guest referee, but was distracted when Young kissed him in response to Venis making out with both Runnels and The Kat. This distraction allowed Moolah to pull The Kat out of the ring, and when Venis saw her out of the ring, he declared Runnels the winner. Post-match, The Kat attacked Runnels by stripping her black body suit off to expose her nude colored thong. The rivalry continued in an arm wrestling match at Insurrextion. They later participated in a "Stink Face match" at SummerSlam, which The Kat won by first performing the move on Runnels.
Runnels later became the onscreen girlfriend and manager of Perry Saturn, who was a member of The Radicalz alliance. She accompanied Saturn to the ring at Fully Loaded in July 2000, where Saturn defeated Eddie Guerrero for the European Championship. Saturn, Runnels, and Radicalz member Dean Malenko also had a series of matches with Team Xtreme (The Hardy Boyz and Lita). The on-screen partnership lasted until Saturn, who had hit his head during a match, opted to continue seeing "Moppy" (a mop he believed was alive) instead of Runnels. As a result, she then became the manager of Raven, a member of The Alliance, who stole Saturn's mop and fed it into a woodchipper.
In late 2001, Terri replaced Trish Stratus as the host of the WWF recap program Excess. She briefly feuded with Stratus, resulting in a wet t-shirt contest between the two on Raw. During this time, she also became the backstage interviewer on Raw and began to wrestle occasionally. On May 27, 2002, she briefly held the perpetually contested Hardcore Championship when she pinned Steven Richards, though he immediately pinned her to regain the title.
After eight years with the company, in March 2004, Runnels was released from WWE.
On January 22, 2018, on the Raw 25 Years special episode, Terri was honored as part of a segment involving women considered legends that contributed to the company's success, including the Bella Twins, Maryse, Kelly Kelly, Lilian Garcia, Torrie Wilson, Michelle McCool, Maria Kanellis and Hall of Famers Jacqueline and Trish Stratus.