Gemignani was raised in Tenafly, New Jersey and graduated from Tenafly High School in 1997. He is a graduate of the University of Michigan's Musical Theater Department. He is the son of Broadway musical director Paul Gemignani and soprano Carolann Page. He is the artistic director of the Eugene O'Neill Theater Center's National Music Theater Conference.
- John Hinckley in the Roundabout Theatre's Assassins
- Beadle Bamford in the John Doyle's 2005 Broadway revival of Sweeney Todd: The Demon Barber of Fleet Street (Drama Desk Award nomination for Outstanding Featured Actor in a Musical)
- Jean Valjean in the 2006 Broadway revival of Les Misérables
- South Pacific, staged concert with Reba McEntire and Brian Stokes Mitchell at Carnegie Hall in June, 2005
- Passion, with Patti LuPone, Michael Cerveris, and Audra McDonald at The Rose Theater in March 2005
- Sweeney Todd in John Doyle's 2007 national tour of the Broadway production opposite Judy Kaye
- Boatman/Dennis in the 2008 Broadway revival of Sunday in the Park with George
- Addison Mizner in the Sondheim-Weidman 2008 musical Road Show at the Off-Broadway Public Theater
- Moishe Rosenwald in the new musical The People in the Picture at Studio 54 in 2011.
- Nick Merritt in the play Headstrong at Ensemble Studio Theatre in 2012
- Billy Flynn in the Broadway revival of Chicago in 2013.
- Violet's Father in the Broadway Revival of Violet.
- King George III in the Chicago production of Hamilton
- Enoch Snow in the Broadway production of Carousel (Tony and Grammy award nominations) in 2018.
- Alfred P. Doolittle in the Broadway production of My Fair Lady in 2019.
In addition, Gemignani participated in a memorable performance of Opening Doors from Sondheim's Merrily We Roll Along at Symphony Space's March, 2005 Wall to Wall Sondheim event celebrating Sondheim's 75th birthday. He also performed in the NY Philharmonic's Gala honoring Sondheim's 80th birthday, singing "Something's Coming" from West Side Story. He performed at the City Center Gala honoring Stephen Sondheim, singing various songs.
Gemignani was married in August 2008 to actor/director, Erin Ortman.
- Kampfe, John. "Tenafly: Small Town, Big Stars – Part 2", Jerseywood, June 13, 2016. Accessed October 9, 2018. "Alexander Gemignani — The son of renowned Broadway music director Paul Gemignani is known primarily for his work on the New York stage.... Gemignani was born in New York City but grew up in Tenafly and, yes, he too is a Tenafly High alum (1997)."
- Hetrick, Adam."Sondheim and Weidman's Road Show Pulls Into the Public Oct. 28" Archived 2008-10-31 at the Wayback Machine, playbill.com, October 28, 2008