Leo Markus, M.D. is Grace's husband. He is a Jewish doctor who works for Doctors Without Borders.
Leo works 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 Grace is to be inseminated with Will's sperm. Will & Grace's plan to become parents together ultimately fails so Leo officially dates Grace and the two become a couple. Grace mentions that Leo gave her a rose on their second date. 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 get married at a spontaneous group ceremony in Central Park, as a publicity broadcast on The Today Show. At the reception, Grace is disheartened to find out how little he knows about Leo. She walks out of the reception and bumps into The Today host Katie Couric, who tells her their wedding is invalid due to the judge not having a proper license. Later that night, however, Grace realizes how much she really wants to be with Leo and agrees to get married at a synagogue in an official wedding. They subsequently move into an apartment in Brooklyn.
Their marriage however, becomes difficult early on 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. At the end of the sixth season, 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.
Leo reappears during season eight, a little over a year after their divorce in "Love is in the Airplane." While on a plane to London, he and Grace have a sexual encounter after talking and 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 but ultimately decides not to after learning from Leo that he has been engaged to another woman, Kate. Will and Grace then decide to raise the baby together.
During the series finale, Leo visits the heavily pregnant Grace to tell her he still loves her. Leo finally finds out that Grace is having his baby and the two of them decide to give their marriage another chance. This causes one final rift in Will and Grace's friendship, which resulted in Grace leaving and joining Leo in Rome where Leo 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.
- Leo's mother calls him Marvin, though it is never mentioned explicitly if this is his real name or a nickname. In "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 has not been cast yet then.
- 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 More
- Marry Me a Little
- 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
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