Megan Mullally Digs Into Her Family's Tragic, Dark History on 'Who Do You Think You Are?' (Exclusive)

The 'Will & Grace' star talks to ET about discovering the hard truths about her family line.

Megan Mullally is chasing down the uncomfortable truths about her family's past.

On Monday's episode of Who Do You Think You Are?, the Will & Grace star travels to Georgia to learn about her family's tragic history, specifically digging into the dark legacy of her father's line. Growing up in Oklahoma City, Oklahoma, Mullally often heard uncouth tales about her ancestors on her father's side that left many questions brimming in her mind. Now, with the help of historians and ancestry experts, Mullally was set on finding the answers.

"A lot of my family has passed away. I didn't know a lot of my family members, and from what I've heard on my father's side, it's just a catastrophe. It's just one big clusterf*ck," Mullally tells ET, sharing that she's hobnobbed with her uncle and a close cousin over family "legend and lore" in recent years. (Watch ET's exclusive sneak peek from Mullally's episode above.) "I thought that was a good jumping-off point."

For Mullally, taking the long road to discovering that painful history through Who Do You Think You Are? was an experience she was ready for. 

"I was really excited to do it, but by the time it came around, I was just in it. I was 100 percent enveloped in the process," she says. "I have to say, for me, finding out the things about my family was surprising, but the real surprising thing to me was the depth and accuracy of the research that they do. I was blown away by the level of expertise of the researchers and historians. That part of it was the most valuable because I felt like it was all completely credible. They had paperwork to back up every aspect of the story as we went along."

In the episode, Mullally receives confirmation -- through historical documents and newspaper clippings -- that the men on her father's side, including her 2nd great-grandfather, have "the same behavioral patterns." "They had problems with addiction, violent tendencies, adultery -- a lot of trends that run through my father's side," the 59-year-old actress says of what she discovered during the process.

One thing she learned that stuck with her was how independent and forward-thinking her great-grandmother, Elizabeth Venable, was for a woman in the late 19th century. "My great-grandmother owned seven properties in Macon [County, Georgia,] which for women at that time was extremely unusual. But she did it because her husband was a ne'er-do-well and she had to support eight children," Mullally recalls. 

It's also revealed in the episode that Mullally's great-grandmother was committed to an asylum for the final seven years of her life, suspiciously after she was sued by her two sons for ownership of her properties. Could her stay at a mental facility have been a ploy by her children? It was a possibility Mullally entertained. "She was a complete badass for her whole life and then all of a sudden, two years after she's sued by sons for the properties and wins the lawsuit, she's committed to an asylum? To me, it seemed a little fishy," she expresses. "I didn't quite buy it."

As Mullally tells it, she's hopeful that she has traces of her great-grandmother's independence and badassery. "I'd like to think so!" she says with a chuckle. "Persevere is one of my words I've always held close to my heart and I've kept plodding along in spite of all odds. I didn't get Will & Grace until I was almost 40. There were times where I thought, 'Should I just go to nursing school?'"

"Now, as I've gotten older, I feel like it's changed into wanting to do more things. I don't want to be limited to just being an actress. I want to direct. We [Mullally and her friend, Stephanie Hunt,] have a band, Nancy & Beth; right now we're making our second record. I'm writing a book with [husband] Nick [Offerman]. There are so many things I would like to do. I would like to think my great-grandmother's gumption has trickled down to me." Mullally paused for a moment before she quipped: "I hope Nick doesn't have me committed!" 

Who Do You Think You Are? airs Mondays at 9 p.m. ET/PT on TLC.