It's hard for me to see a definitive lead since IAL was really an ensemble show and had to go through numerous retools and cast departures during its six season (nine year!) run. In the first season, Susan Sullivan (Lois) was first billed. In the second season (as Making a Living), Louise Lasser (Maggie) was first billed. When IAL was revived for the syndication market, Ann Jillian (Cassie) was first billed and then for the last three seasons (after Jillian left), Barrie Youngfellow (Jan) was first billed.

It's easy to assume that since Barrie Youngfellow stayed with the show from start to finish and was eventually first billed, Jan was by default, the main character. On the same token however, Gail Edwards (Dot) was the only other waitress who stayed on IAL from beginning to end. And the very last episode ("The Dot and Sonny Show") is centered on her character.

Then again, Ann Jillian despite only being a cast member for the first three seasons, is quite obviously the "face" (or at the very least, the breakout star) of the show. The publicity behind Jillian's breast cancer ordeal apparently played a huge factor in production for IAL resuming (three years after it ended its run on ABC) for the 1985-86 season.

either Maggie, Cassie or Lois

I was reading the comments section for a video on Jennifer Slept Here (https://www.youtube.com/watch?v=55Bi23sSMtM), which was a show on NBC that Ann Jillian appeared in during the interim of of It's a Living being cancelled by ABC and later revived for the syndication market.

One of the comments pointed out that even though IAL was an ensemble, Ann Jillian was certainly the star of the show. In Season 2 when it was officially titled Making a Living, they added Louise Lasser (formally of Mary Hartman Mary Hartman), who didn't quite fit in. The other characters never really made an impression outside of Cassie.

And Sonny the piano player, who was meant to the the comic relief character was really more obnoxious (he was kind of like Dwayne Schneider on One Day at a Time, where I guest he was meant to be a chauvinist acting foil to the female characters) than he was funny.

After some thought, I'm quite certain now that Susan Sullivan was in effect, the lead or the actress that the show was built around. She at the very least, seemed like she was meant to be the show's "glue" and moral center.

In the second season, when it was retooled as Making a Living, Louise Lasser, about four years after the the demise of Mary Hartman, Mary Hartman, became the de facto lead.

When It's a Living was brought back for first-run syndication three years later, Ann Jillian officially rose to the lead, because by then, she'd already been the star of her own sitcom--the short-lived Jennifer Slept Here.

It was then in, Season 4 going forward when Barrie Youngfellow rose to the top.