Alexander performed a mentalism and magic act at The Magic Castle in Hollywood, California, from April 24 to 30, 2006, and was later named The Academy of Magical Arts Parlor Magician of the Year for this act. He won the Academy’s Junior Achievement Award in 1989.

Alexander voiced the character Abis Mal in The Return of Jafar and the TV series based on the 1992 film Aladdin. In 2009, he played Joseph in the Thomas Nelson audio Bible production The Word of Promise. The project featured a large ensemble of actors, including Jim Caviezel, Lou Gossett Jr., John Rhys-Davies, Jon Voight, Gary Sinise, Christopher McDonald, Marisa Tomei and John Schneider.

Alexander grew up in Livingston, New Jersey, and is a 1977 graduate of Livingston High School. Interested in magic from an early age, Alexander initially hoped to be a magician, but while attending a magic camp was told that his hands were too small to palm a card, making card magic virtually impossible. He then became interested in theater, eventually coming to realize, “Wait a minute—the whole thing’s an illusion. Nothing up there is real” and that theater was “a magic trick”. He then decided to pursue it as a career. He attended Boston University, but left the summer before his senior year after getting work in New York City. At Boston University, Alexander wanted to pursue classical acting, but a professor redirected him toward comedy after noticing his physique, remarking, “I know your heart and soul are Hamlet, but you will never play Hamlet.” He was awarded an honorary degree in 1995.

Alexander’s second chance as a TV series lead, the CBS sitcom Listen Up! (2004–05), also fell short of a second season. Alexander was the principal executive producer of the series, based very loosely on the life of the popular sports-media personality Tony Kornheiser. Alexander appeared on the Family Guy: Live in Vegas CD and sang a verse in a song. He was featured in the Friends episode “The One Where Rosita Dies” as Earl, a suicidal supply manager. Phoebe calls him trying to sell him toner, learns about his problem, and tries to persuade him not to commit suicide. This is referenced in an episode of Malcolm in the Middle where Alexander appears as Leonard. a neurotic and critical loner. He describes himself as “free” and says he makes money “selling toner over the phone”. Later in the episode, he is repeatedly harassed by a man named George. Alexander appeared in the 1995 TV version of the Broadway musical Bye Bye Birdie, as Conrad Birdie’s agent, Albert Peterson. He guest-starred in episode 8 of the 1996 variety show Muppets Tonight.

Alexander starred in several commercials during the 1980s. Among them were commercials for Hershey’s Kiss; Delta Gold potato chips; Miller Lite beer; McDonald’s McDLT hamburger; Pabst Blue Ribbon beer; Levi’s 501 jeans; Sony Watchman TV; and Western Union wire transfer. In January 1995, he did a commercial for Rold Gold pretzels to be broadcast during the Super Bowl. The commercial depicts him with Frasier dog Eddie jumping out of an airplane with a parachute over the stadium. After the commercial, the audience is brought back to a supposedly live feed of the playing field hearing startled sports commentators as Alexander and the dog land in the field to wild applause. He appeared in Kentucky Fried Chicken (KFC) commercials in 2002, including one with Barry Bonds of the San Francisco Giants and another with Trista Rehn of The Bachelorette. It was rumored that he quit doing these commercials due to KFC suppliers and slaughterhouses’ alleged cruelty to animals, but he denied that in the August 2, 2006, issue of Adweek, saying, “That’s PETA bullcrap. I loved working for KFC. I was targeted by PETA to broker something between them. I think KFC really stepped up to the plate; unfortunately PETA did not.” In 2007, Alexander appeared in a commercial for the ASPCA that aired on cable TV stations. In 2018, Alexander became one of several celebrities to play Colonel Sanders in commercials for KFC, reprising his role from the 2002 campaign.

In addition to his roles as an insensitive, money-hungry lawyer in Pretty Woman and as inept womanizer Mauricio in Shallow Hal, Alexander has appeared in Love! Valour! Compassion!, Dunston Checks In, Love and Action in Chicago, The Last Supper and Jacob’s Ladder. He voiced the gargoyle Hugo in Disney’s 1996 animated film The Hunchback of Notre Dame and its direct-to-video sequel, The Hunchback of Notre Dame II. His other Disney voice work includes House of Mouse and the video game Kingdom Hearts 3D: Dream Drop Distance. He has dabbled in directing, starting with 1996’s For Better or Worse and 1999’s Just Looking. He also played the toymaker A.C. Gilbert in the 2002 movie The Man Who Saved Christmas. In 2009, Alexander had a small role in the movie Hachi: A Dog’s Tale as a train station manager. He starred as Cosmo in A Fairly Odd Movie: Grow Up, Timmy Turner!.

In 1999, Alexander presided over the New York Friars Club Roast event honoring Jerry Stiller, who played his father on Seinfeld; it also featured appearances by Kevin James and Patton Oswalt, both Stiller’s costars on The King of Queens.

Alexander made cameo appearances as himself in the second season of Curb Your Enthusiasm, and appeared in the seventh season of Curb Your Enthusiasm along with the other three principal Seinfeld cast members. He had a part in the ABC sitcom Dinosaurs as Al “Sexual” Harris (who frequently engaged in sexual harassment), as well as other voices. Despite a successful career in film and stage, Alexander never managed to repeat his Seinfeld-level of success in television. 2001 marked his first post-Seinfeld return to prime-time television: the heavily promoted but short-lived ABC sitcom Bob Patterson (which was canceled after five episodes). Alexander partially blames the show’s failure on the country’s mood after 9/11.

Alexander began his acting career on the New York stage and is an accomplished singer and dancer. On Broadway he appeared in Stephen Sondheim’s Merrily We Roll Along, Kander & Ebb’s The Rink, Neil Simon’s Broadway Bound, Accomplice, and Jerome Robbins’ Broadway, for which he garnered the 1989 Tony Award for Best Leading Actor in a Musical. In 2003, Alexander was cast opposite Martin Short in the Los Angeles production of Mel Brooks’s The Producers. Alexander also appeared with Kelsey Grammer in the 2004 musical adaptation of Charles Dickens’s A Christmas Carol, as Jacob Marley. He continues to appear in live stage shows, including Barbra Streisand’s memorable birthday party for Sondheim at the Hollywood Bowl, in which he performed selections from Sweeney Todd: The Demon Barber of Fleet Streetwith Angela Lansbury. Alexander was recently named the artistic director of Reprise Theatre Company in Los Angeles, where he previously directed Sunday in the Park with George. He is scheduled to direct its upcoming revival of Damn Yankees. In 2015, he replaced Larry David as the lead in David’s Broadway play Fish in the Dark. Alexander co-starred opposite Sherie Rene Scott in the September 2017 world premiere of John Patrick Shanley’s The Portuguese Kid at the Manhattan Theatre Club.

Alexander appeared in the Star Trek: Voyager episode “Think Tank” as Kurros, a genius alien trying to get Seven of Nine to serve on his ship. He appeared in “One Night at Mercy”, the first episode of the short-lived 2002 revival of The Twilight Zone, playing Death. He featured in the 2005 Monk episode “Mr. Monk and the Other Detective” as Monk’s rival, Marty Eels. On the June 26, 2006, episode of Jimmy Kimmel Live!, Alexander demonstrated several self-defense techniques. He hosted the July 4, 2006, PBS “A Capitol Fourth” celebrations in Washington, D.C., singing, dancing, and playing tuned drums. In 2006, Alexander signed on to feature as a regular cast member in the second season of Everybody Hates Chris. He hosted the Comedy Central Roast of William Shatner on August 13, 2006 (first airdate: August 20, 2006). In 2007, Alexander was a guest star in the third episode of the improv comedy series Thank God You’re Here. He has been a frequent guest and panelist on Bill Maher’s Politically Incorrect and Real Time; Hollywood Squares; the Late Late Show, with both Craig Kilborn and Craig Ferguson; and the Late Show with David Letterman.

More recently, Alexander has competed on televised poker shows and in various tournaments. He appeared twice on Bravo’s Celebrity Poker Showdown, winning the final table of the 8th season. Alexander won the $500,000 prize for the charity of his choice, The United Way of America, to help benefit the New Orleans area. Alexander played in the 2007 World Series of Poker main event, but he was eliminated on the second day. He returned in 2009, making it to day 3 of the event and finishing in the top 30% of the field. Alexander has appeared on NBC’s Poker After Dark in the “Celebrities and Mentors” episode, finishing in 6th place after being eliminated by professional poker player Gavin Smith. He signed with PokerStars, where he plays under the screen name “J. Alexander”.

In 2008, Alexander guest-starred in the season four episode “Masterpiece” of the CBS show Criminal Minds as Professor Rothschild, a well-educated serial killer obsessed with the Fibonacci sequence who sends the team into a race against time to save his last victims. He returned in the same season to direct the episode “Conflicted”, featuring the actor Jackson Rathbone. In 2011, Alexander was the guest star in an episode of Harry’s Law, playing a high school teacher bringing a wrongful dismissal suit.

Alexander hosted the LOL Sudbury opening night gala in Sudbury, Ontario, Canada on May 29, 2008, which was simulcast throughout Canada at 60 Cineplex theaters, a first for any comedy festival. He has lent his voice to several episodes of the Twilight Zone Radio Dramas.

In 2008 and again in 2009, Alexander fronted Jason Alexander’s Comedy Spectacular, a routine exclusive to Australia. The show consists of stand-up and improvisation and incorporates Alexander’s musical talent. He is backed up by a number of well-known Australian comedians. His first time performing a similar show of this nature was in 2006’s Jason Alexander’s Comedy Christmas. In February/March 2010, Alexander starred in his own show, The Donny Clay Experience, at the Planet Hollywood Resort in Las Vegas, Nevada. Donny Clay, whom he has portrayed in a tour of the United States, is a self-help guru in a similar mold to his Bob Patterson character.

Alexander was the national spokesman for the Scleroderma Foundation, a leading organization dedicated to raising awareness of the disease and assisting those who are afflicted. On January 6, 2010, it was announced that he would be the new face of the weight loss company Jenny Craig. In summer 2005, he appeared with Lee Iacocca in ads for DaimlerChrysler. Iacocca did the ads as part of a way to raise money for Denise Faustman’s research on autoimmunity. Iacocca and Alexander both have loved ones whose lives have been adversely affected by autoimmunity.

Alexander has been a prominent public supporter of the OneVoice initiative, which seeks out opinions from moderate Israelis and Palestinians who want to achieve a mutual peace agreement. On Real Time with Bill Maher he said he had visited Israel many times and spoke about progress toward peace he had observed. In 2012, Alexander announced his support for President Barack Obama’s reelection.

In 2018, Alexander played Olix the bartender in The Orville. The same year, he portrayed Gene Lundy, a drama teacher, on two episodes of Young Sheldon.

In 2019, Alexander appeared on The Marvelous Mrs. Maisel as Asher Friedman, a blacklisted Broadway playwright who is an old friend of Midge Maisel’s father Abe Weissman.

In 2020, Alexander hosted the Saturday Night Seder, an online Passover Seder that featured many celebrities and benefited the CDC Foundation.