Casting
Actors Bonnie Franklin, Pat Harrington Jr., and Valerie Bertinelli were the only cast members to remain with the series throughout its entire run. Lead actress Mackenzie Phillips was fired after the fifth season due to growing problems with substance abuse. She later returned in a frequent recurring role, but was let go again shortly after the start of the final season. Original cast member Richard Masur was written out early in the second season, but returned as a guest star in the sixth-season finale.
Richard Masur played attorney David Kane, a love interest for the newly divorced Ann Romano, in the first season and left after the fourth episode of the second season. In a 2016 interview with The A.V. Club, he explained that David was constantly proposing marriage to Ann and she refused him every time. The actor became even further frustrated when Lear insisted that David and Ann's romance be unconsummated. His role was relegated to being a confidante to Julie and Barbara and an adversary to Schneider. After several disagreements with the direction Masur's character took, Lear agreed to write off David from the show but not entirely, per his appearance in the finale.
After Masur's departure, the producers replaced the romantic interest with a comedic foil. That role was filled by actress and comedienne Mary Louise Wilson, playing as Ginny Wroblicki, a cocktail waitress who becomes Ann's neighbor, best friend, and confidante. It was an unhappy casting change all around, as the show's ratings began to decline soon after Wilson's arrival; the character of Ginny Wroblicki proved to be unpopular with viewers, and Wilson herself did not like the role or get along with Franklin.
Wilson wrote that "aside from Lear, nobody thought I was funny...To make matters worse, each character, according to the show's formula, had to have a 'serious' moral dilemma at some point, and I was given some problem about an illegitimate child to work out in these increasingly sentimental scenes that made my bowels shrink." At the end of the second season of One Day at a Time, Wilson was released from the show at her own request after appearing in 14 episodes. The character of Ginny Wroblicki was never seen, referred to, or heard from again except in a fifth-season retrospective clip episode.
For the next two seasons, the central cast of Ann, Julie, Barbara, and Schneider was supplemented by recurring characters, including William Kirby Cullen as Julie's boyfriend Chuck Butterfield, Howard Morton and K Callan as Chuck's parents, John Putch as Barbara's awkward friend Bob Morton, Scott Colomby as Barbara's boyfriend Cliff Randall, and John Hillerman and Charles Siebert as Ann's bosses, Mr. Connors and Mr. Davenport, respectively. Dick O'Neill and Nedra Volz made three appearances together as Orville and Emily, residents of the retirement home where the main characters put on a semiregular variety show. Joseph Campanella also made several appearances as Ann's ex-husband and the girls' father, Ed Cooper.
Michael Lembeck joined the series as Julie's husband, Max, in the fifth season, but he was written out as a consequence of Phillips' firing, but later returned in season seven along with Philips. A steady stream of regulars was added in the ensuing seasons, including Ron Rifkin as Ann's boyfriend, Nick; Glenn Scarpelli as Nick's son, Alex; and Boyd Gaines as Barbara's boyfriend, later husband, Mark. The character of Nick lasted only one season, being killed off in a car crash, but Nick's son Alex stayed with the family for the following two years.
Shelley Fabares, who had previously guest-starred as Ann's rival co-worker Francine Webster, appeared more frequently, eventually becoming a regular. Nanette Fabray, who played Ann's mother, also made more frequent appearances before becoming a regular cast member in the final season. Howard Hesseman joined the series for a short time as Mark's father, Sam, who would become Ann's second husband.
The character of Julie was written out again in the ninth season, as Mackenzie Phillips' erratic behavior and drug-related issues recurred. She was dropped from the opening credits and not present after the fourth episode of season 9, although for a time she was still referred to as an off-screen character, while Michael Lembeck was still seen regularly as Julie's husband, Max. Their daughter Annie was also seen. A few episodes later, Julie—still unseen—left Max, writing him a "Dear John" letter. The character of Max stuck with the show, but Julie was not seen again.
Notable guest stars throughout the series run include Norman Alden, Robby Benson, Carla Borelli, Charlie Brill, Dennis Burkley, Jack Dodson, Elinor Donahue, Gwyda Donhowe, David Dukes, Greg Evigan, Conchata Ferrell, Corey Feldman, Alice Ghostley, Lee Grant, Mark Hamill, Jim Hutton, Van Johnson, Terry Kiser, Richard Kline, Christopher Knight, Jay Leno, Robert Mandan, Robert Morse, Denise Nicholas, J. Pat O'Malley, Jo Ann Pflug, Eve Plumb, Susan Richardson, William Schallert, Gretchen Corbett, Suzanne Somers, Ellen Travolta, Dick Van Patten, and Keenan Wynn.
= Main cast (credited)
= Recurring cast (3+)
= Guest cast (1–2)