Leo Markus, M.D. is Grace's ex-husband. He is a Jewish doctor who works for Doctors Without Borders.
Leo grew up in Baton Rouge, Louisiana and he mentions that he lost his Cajun accent early on. He received his medical degree at NYU Med School and subsequently joined Doctors Without Borders.
Leo is working as a doctor in New York when he meets Grace. After Grace regains consciousness for hitting her head on a lamp post, Leo takes her to the fertility clinic where she is to be inseminated with his best friend Will's sperm. Will and Grace's plan to become parents together ultimately fails so Leo and Grace start dating. Grace mentions that Leo gave her a rose on their second date, after which she felt bad not getting anything for him, so she slept with him.
Their relationship initially becomes a constant source of tension with Will, although they eventually achieved a certain accord with each other, Leo even commenting that marrying Grace entails marrying Will as well.
In November 2002, Grace and Leo spontaneously decide to get married at a group ceremony in Central Park, as a publicity broadcast on The Today Show. However, at the reception, Grace is disheartened to find out how little they actually know about each other and walks out. Exacerbating things, they later find out that the wedding is invalid due to the judge not having a proper license. That night, however, Grace realizes how much she really wants to be with Leo and they decide to get married at a synagogue in an official wedding. They spend their honeymoon at the Carribean  and subsequently move into an apartment in Brooklyn.
Their marriage however, becomes difficult as Leo is called to work for Doctors Without Borders again and leaves the country frequently. Grace goes with him to Cambodia but returns soon after she realizes her marriage is in trouble.
During Karen and Lyle's wedding, Leo returns to New York and admits to having a one-night stand with another doctor while in Cambodia. Grace tries to forgive him but they divorce soon after and she moves back in with Will.
A little over a year after their divorce, Leo and Grace bump into each other on a plane to London, seemingly rekindling their romance. Grace later finds out that she is pregnant because of this encounter. She meets with Leo at a cafe to tell him about her pregnancy but ultimately decides not to after learning from Leo that he has been engaged to another woman named Kate. Will and Grace then decide to raise the baby together.
During the finale, Leo visits Grace to tell her he still loves her, hoping to give their marriage another try. Leo finally finds out that Grace is having his baby and she accepts him. This causes one final rift in Will and Grace's friendship, which resulted in Grace leaving for Rome with Leo where he now works as a hospital researcher. After giving birth to a daughter they named Lila, they move back to Brooklyn, New York to raise their family.
As with much of the events during season 8, Grace's pregnancy with her and Leo's child are revealed to be just Karen's dream during the season 9 premiere. He and Grace are still divorced and are now living separate lives. Grace however finds out that Leo is still listed as his emergency contact when she undergoes a biopsy and he is contacted by the clinic. Although bitter at first, Grace realizes life is too short and makes peace with Leo.
- Leo's mother calls him Marvin, though it is never mentioned explicitly if this is his real name or a nickname. In the episode FYI: I Hurt Too, it is mentioned that his medical diploma says "Dr. Leo Markus".
- Leo and Grace's wedding marks the first time in television history that a Jewish protagonist character had married inside the faith.
- Leo's silhouette appear at the end of season 4 but his face is not clearly seen until the premiere episode of season 5. The silhouette previously shown is not of Harry Connick, Jr. as he had not been cast yet.
- A.I.: Artificial Insemination
- ...And the Horse He Rode in On
- Bacon and Eggs
- The Kid Stays Out of the Picture
- It's the Gay Pumpkin, Charlie Brown
- The Needle and the Omelet's Done
- Marry Me a Little, Marry Me a Little More
- All About Christmas Eve
- Fagmalion Part 2: Attack of the Clones
- Leo Unwrapped
- Dames at Sea
- Last Ex to Brooklyn
- Home Court Disadvantage
- A-Story, Bee-Story
- I Do, Oh, No, You Di-in't
- FYI: I Hurt, Too
- Saving Grace, Again: Part 1
- Love Is in the Airplane
- Grace Expectations
- The Finale
- Emergency Contact
You know, all I could think about since I left was one of your home cooked meals... so let's go upstairs and have sex. — Dames at Sea
If I were replacing an aortic valve, I could do it with my eyes closed. But this... this is delicate! It's brunch! — The Needle and the Omelet's Done
- ↑ 1.0 1.1 Emergency Contact
- ↑ 2.0 2.1 Back Up, Dancer
- ↑ 3.0 3.1 FYI: I Hurt, Too
- ↑ ...And the Horse He Rode in On
- ↑ Humongous Growth
- ↑ 23
- ↑ Marry Me a Little, Marry Me a Little More
- ↑ The Honeymoon's Over
- ↑ Fagmalion Part One: Gay It Forward
- ↑ Women and Children First
- ↑ Swimming from Cambodia
- ↑ I Do. Oh, No, You Di-in't
- ↑ Love is in the Airplane
- ↑ The Definition of Marriage
- ↑ The Finale