Tillmann was born in Marshall, Minnesota, on February 6, 1978 at Weiner Memorial Hospital. In 1987, his family moved to Owatonna, Minnesota. He graduated from the Perpich Center for Arts Education in Golden Valley, Minnesota. His family is still based in Owatonna, where his mother and father work as educators.

In the Season 3 premiere of Broad City, Har Mar played hipster gallery patron "Oy Bomb," a reference to the similarly named Soy Bomb who crashed Bob Dylan's 1998 Grammy performance.

Over the years, Sean Tillmann has performed on several television shows, including The Sharon Osbourne Show, Late Night with Jimmy Fallon, and Jimmy Kimmel Live! in the United States. In the United Kingdom, he has appeared on Channel 4 music programme Born Sloppy, Bo' Selecta!, and Friday Night with Jonathan Ross. In 2003 and again in 2014, he was a guest on Never Mind the Buzzcocks.

He made a cameo in the 2004 movie Starsky and Hutch as 'Dancing Rick', a disco dance champion who faces off in a contest with Ben Stiller's character.

In addition to his own headlining tours, Har Mar has opened for Ben Lee, The Strokes, Incubus, Tenacious D and the Yeah Yeah Yeahs. In 2007, Har Mar Superstar supported the Red Hot Chili Peppers on their tour of Australia and New Zealand. Fabrizio Moretti (The Strokes) and Tony Bevilacqua (ex-Distillers) have served as Har Mar's rhythm section, playing drums and bass guitar respectively. Har Mar supported Sia Furler on her 2008 tour with Denver Dalley on bass and Nick Slack on drums. Other bassists have included Jenni Tarma, Jeff Quinn, and John Fields, while Bert Thomas, Jeff Brown and Michael Bland have manned the drum stool.

In 2008, Har Mar Superstar was part of the live touring set-up of Neon Neon: the band created by Super Furry Animals' Gruff Rhys and producer/solo Artist Boom Bip. Har Mar sings plus plays percussion during live shows and is featured on the recorded version and video of "Trick for Treat": a song on the band's debut album, Stainless Style, which was nominated for the Mercury Prize in 2008. Har Mar also performed backing vocals and appeared in the video for "Catch My Disease" by Ben Lee.

On October 26, 2009, Har Mar Superstar was the musical guest on Late Night with Jimmy Fallon and earned rave reviews for his performance of "Tall Boy," immediately increasing his popularity within North America as noted in an article by MTV.

In February 2011, Har Mar started recording and performing with the soft rock-inspired band Gayngs and experimental rock group Marijuana Deathsquads. Har Mar and Marijuana Deathsquads teamed up for month-long residencies in Los Angeles at Spaceland (February 2011), in New York City at Bowery Electric (October 2011) and in Austin, Texas (February 2012).

On September 10, 2012, Earwolf released Har Mar's inaugural podcast, Nocturnal Emotions. Each week Har Mar has hour-long conversations with a different musician, actor, or Artist. Regular features include embarrassing moments, "Let Me Ruin Your Favorite Song," & "Let's Create a Show." Guests have included Ellen Page, Macaulay Culkin, Alia Shawkat, Adam Green, Flea, Father John Misty, Jon C. Daly, MNDR, Gavin McInnes, Patrick Carney, Jake Fogelnest, Wyatt Cenac, David Yow, Mae Whitman, Jenny Eliscu, Jonah Ray, P.O.S, Channy Leaneagh, Randy and Jason Sklar, Jena Friedman, Jesse Pearson, Ted Leo, Matty McLoughlin, Matt Sweeney, Jenn Robinson, Jared Swilley, Bridey Elliott, Jesse Pearson, Paul Sprangers, Aaron Pfenning, Steve Agee, Harris Wittels, Cedric Bixler-Zavala, Dave Hill, Caveman, Tanlines, Doug Benson, Matt Mira, Simon O'Connor, Jack Dishel, Colin Hanks, James Levy, Ryan Jarman, Albert Hammond, Jr., and Julian Casablancas. Nocturnal Emotions is available for free on Earwolf.com and iTunes.

R.T. Rybak, then mayor of Minneapolis, MN declared Friday, September 20, 2013, to be “Har Mar Superstar Day.” Sean Tillmann was presented with an official Proclamation from the city in recognition of his achievements in music and film, and in light of his continued connection to Minneapolis.

In October 2014, Har Mar returned to Never Mind the Buzzcocks alongside Nicole Scherzinger, Kerry Godliman & James Acaster.

Collaborating once again with Julian Casablancas, Tillmann released his sixth Har Mar album – Best Summer Ever—on April 15, 2016. Best Summer Ever, since its inception, has been imagined as a "Greatest Hits of Har Mar Superstar from 1950–1985," even though Tillmann was born in 1978. Touring the United States & Europe in support of the new album, Har Mar has enlisted six other Musicians to bring the songs to life.